Recipe: Sweet Potato Gratin with Caramelized Onions (2024)

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Faith Durand

Faith DurandSVP of Content

Faith is the SVP of Content at Apartment Therapy Media and former Editor-in-Chief of The Kitchn. She is the author of three cookbooks, including the James Beard Award-winning The Kitchn Cookbook. She lives in Columbus, Ohio, with her husband and two daughters.


updated Jan 29, 2020


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Recipe: Sweet Potato Gratin with Caramelized Onions (1)


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Since Thanksgiving is virtually around the corner, and now is the time for orange vegetables to assert their place, I felt it was high time to re-introduce you to my favorite sweet potato gratin recipe. This is a smoky, rich gratin, with a streak of caramelized onions and garlic sandwiched between layers of sweet potatoes baked in cream. I hate to sound like I am bragging, but just to put it in perspective, every time I have served this, grownups have literally licked their plates and groaned aloud.

What I can say? It’s the power of sweet potatoes in cream.

This gratin also has a little kick to it, since I have never understood why people add so much sugar to sweet potatoes. With the exception of desserts like sweet potato pie, it seems wholly unnecessary. Instead of marshmallows or brown sugar crumble, I top this gratin with a crisp layer of breadcrumbs with bit of cheese for savor, and I add caramelized onions for their own sort of sweetness and richness, and pinch of chipotle powder for a slow, warm heat that balances the sweetness.

This dish is not for dieters; it would be a dessert too if not for the sage and chipotle that give it fullness and a kick. Unless you cannot tolerate any heat at all, do use the chipotle; it adds a warmth that balances the richness of sweet potatoes, layered with cream and caramelized onions under a crunchy Parmesan crust.

The gratin in process: Spreading caramelized onions over the first layer of sweet potato slices.


Sweet Potato and Sage Gratin

Serves 6

Nutritional Info


  • 2 tablespoons

    unsalted butter

  • 2

    medium onions, peeled and sliced into thin half moons

  • 3 pounds

    sweet potatoes or yams (3 to 4 large potatoes)

  • Kosher salt and freshly ground black pepper

  • 4 cloves

    garlic, minced

  • 1/4 cup

    fresh sage leaves, finely chopped

  • 1/4 teaspoon

    chipotle pepper powder (optional)

  • 1 cup

    heavy cream

For topping:

  • 1 tablespoon

    unsalted butter

  • 2 tablespoons

    finely chopped sage leaves

  • 1/2 cup

    fine dried breadcrumbs

  • 1/2 cup

    grated Parmesan

  • Olive oil


  1. Heat the oven to 350°F and lightly grease a 9x13 casserole or gratin dish with olive oil or butter. Heat the butter in a heavy skillet (cast iron, ideally) and when it foams up, add the onions. Sprinkle them lightly with salt. Slowly caramelize the onions over medium heat, stirring frequently. This will take at least 15 minutes.

  2. While the onions are caramelizing, peel and slice the potatoes in thicknesses of about 1/4-inch. Tile half of them in a the prepared casserole dish, overlapping them in tight rows or a spiral. Sprinkle them lightly with salt and pepper.

  3. When the onions are dark brown, add the garlic and cook just until golden. Finely chop the sage and add this to the onions, along with the chipotle powder and cream. Bring to a simmer and cook until the cream is slightly reduced then remove from the heat.

  4. Use a slotted spoon to remove the onions and garlic from the cream. Spread the onions over the layer of sweet potatoes in the dish. Layer the remaining potatoes on top, forming a tight spiral or rows. Sprinkle lightly with kosher salt and black pepper. Pour the cream over the top. It will look like there isn't enough cream; it won't cover the potatoes or fill up the dish. Resist the impulse to add more cream, however; there is plenty to cook the potatoes, and adding more may cause the potatoes to get mushy and fall apart.

  5. Bake uncovered for 25 to 30 minutes or until the potatoes are just tender and the cream is golden brown and bubbling. (Note: Baking time is highly dependent on how thick you cut your potatoes. The 25 to 30 minute time is based on 1/4-inch thick slices. Any thicker and the baking time may rise.)

  6. Meanwhile, heat the remaining tablespoon of butter in the skillet and toast the reserved chopped sage and the breadcrumbs until golden and fragrant. Remove from the heat and toss with the Parmesan. Sprinkle this over top of the gratin and drizzle the top lightly with olive oil. Return to the dish to the oven for about 15 minutes or until brown and crispy. Let cool for at least 15 minutes before slicing and serving.

Recipe Notes

Gruyere is another cheese that is excellent in the breadcrumb topping.

This recipe makes a full 9x13-inch dish, but there are only two layers of potatoes. If you would like to maker a deeper gratin and increase the amount of servings, double the onions and increase the potatoes by 50%. Then create a second layer of caramelized onions and sweet potatoes on top of the first. Increase the cream by 50% as well, and expect to bake at least 10 minutes longer.

More Sweet Potato Recipes:
Recipe: Sweet Potato Gratin with Smoky Breadcrumbs
Recipe: Sweet Potato Soup with Miso and Ginger

(Images: Faith Durand)

(Originally published March 7, 2007)

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Recipe: Sweet Potato Gratin with Caramelized Onions (2024)


Can I store onions and sweet potatoes together? ›

What you don't want is to have your potatoes and onions in close proximity, as gases from the onions can hasten sprouting in potatoes. Kept in the dark: Davison says your potatoes should be stored inside a paper bag in a cool, dark, dry place.

Why is my sweet potato casserole soupy? ›

(Another hot tip if you're boiling your sweet potatoes: Don't overcook them! They tend to absorb water, which could make the filling watery.)

Can you store russet potatoes and sweet potatoes together? ›

Sweet potatoes prefer slightly different conditions that white potatoes. Around 55 to 60 degrees Fahrenheit with some humidity is best. They'll still require a dark place away from other fruits and vegetables, except maybe an apple.

Why can't you store onions and potatoes together? ›

But, here's the catch: Storing potatoes and onions together will drastically shorten their shelf life. The reason? Onions produce ethylene gas, a gas that causes potatoes to spoil prematurely. Conversely, potatoes' high moisture content can cause onions to turn brown and mushy.

What is the best container to store potatoes and onions in? ›

Two-Tier Bamboo Baskets

Declutter your produce collection in style with this two-tier bamboo basket. It offers ventilation from three different angles and enough capacity to store up to seven pounds of fruit, veggies, onions and potatoes on the kitchen counter or pantry shelf.

What brings out the flavor of sweet potatoes? ›

The potatoes themselves get a rich, almost toffee-like complexity due to the slow roasting. Adding butter that's been cooked down until its milk solids just begin to turn brown and nutty accentuates that flavor even more. Some folks like to add cinnamon, nutmeg, or other spices to their sweet potatoes.

Does cooking a sweet potato longer make it softer? ›

The potato pulled at 1 hour reached about 180 degrees, the 2-hour potato reached about 190 degrees, and the 3-hour potato reached about 200 degrees. Only the potato that cooked for the full 3 hours boasted the creamy, soft consistency and caramelized sweetness I wanted.

Why do Americans put marshmallows on sweet potato casserole? ›

Most people point to a 1917 pamphlet by the Angelus Marshmallow Company as the first time sweet potatoes and marshmallows came together. This company is believed to be the first to commercially-produce and sell the squishy white marshmallows we know, roast, and snack on today.

What is the sticky liquid from sweet potato? ›

The ooze is actually referred to as "sap." It is made up of sugar and starch combined with the moisture found in the vegetable, and it escapes out of the damage made from the knife. The sap really equals the sweetness. If it's pouring out of your potato, that means it's a sweet and delicious vegetable.

Can you overcook a sweet potato? ›

Mushy Texture: Sweet potatoes contain a lot of moisture, and overcooking can cause them to become mushy and overly soft. Instead of a firm and slightly tender texture, they may turn into a puree-like consistency. Loss of Flavor: Overcooking can lead to a loss of flavor in sweet potatoes.

When should you throw out sweet potatoes? ›

How to tell if sweet potatoes have gone bad. If your sweet potato is soft in spots, smells rotten, or oozes a mysterious liquid, that potato should be discarded. Another sign that sweet potatoes have taken a turn for the worse is if they start growing stalky purplish sprouts.

What happens if you leave sweet potatoes in the ground over winter? ›

Sweet potato roots continue to grow until frost kills the vines. Roots can be left in the ground for a short while; however, a hard frost can cause damage to roots near the surface. Chilling injury also results to roots when soil temperatures drop to 50°F or lower, and this can result in internal decay in storage.

How many days can you eat leftover sweet potatoes? ›

Cooked sweet potatoes generally keep without spoiling for a few hours when stored at room temperature. They can last three to five days if stored in an airtight container and refrigerated. Frozen cooked sweet potatoes will keep well for up to six months.

How far apart should onions and potatoes be stored? ›

We've always kept our onions and potatoes in the same pantry … the potatoes are in a ventilated bin on the floor, while the onions are in a ventilated basket about three shelves above the potatoes … more specifically, the onions are being stored about 3.5 feet from the potatoes.

Which vegetables should not be stored together? ›

  • Keep Onions Away From Apples and Potatoes.
  • Store Melons Solo.
  • Keep Cauliflower Away From Apples, Kiwis, and Onions.
  • Place Mushrooms Away From Pungent Foods.
  • Use Your Crisper Drawer.
  • Separate Ethylene Producers and Ethylene Sensitive Produce.
Aug 5, 2022

Can you store potatoes and onions on the counter? ›

Onion bulbs and potato tubers both flourish underground, meaning that the vegetables require similar storage conditions — cool, dark, and ventilated environments — and they fare much better on a counter than in the fridge.

What's the best way to store sweet potatoes? ›

Choose A Dark Place

Instead, keep your sweet potatoes in a paper bag or basket in a kitchen cabinet, pantry, or basem*nt. Choose a cabinet that's not near the heat of your oven. A spot closer to the floor will be cooler. Sweet potatoes prefer a temperature between 50 and 60 degrees.

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