Sweet potato gratin is a creamy, baked side dish that’s simple to prepare and full of flavor. It makes an excellent Thanksgiving or holiday side, but it’s easy enough for everyday meals. With layers of thinly sliced sweet potatoes coated in a garlic-parmesan cream sauce, this gratin pairs well with meat, chicken, seafood, or vegetarian mains.
This gratin is made with a few straightforward ingredients and baked until tender and golden. The layers of sweet potato are lightly seasoned and finished with freshly grated Parmesan and a velvety cream sauce flavored with garlic and onion.
For more gratin ideas, try the Sweet Potato and Kale Gratin or Parmesan Zucchini and Tomato Gratin.
Ingredients for Sweet Potato au Gratin
- Sweet potatoes: A good source of vitamins and minerals, including vitamin A, vitamin C, B6, potassium, and magnesium.
- Butter: Salted or unsalted works; adjust added salt accordingly.
- Onion and garlic: Sautéed to bring sweetness and depth to the sauce.
- Cream: Heavy whipping cream gives the sauce its rich, silky texture.
- Arrowroot starch (or cornstarch): Used to thicken the cream sauce.
- Lemon juice: Fresh lemon brightens the flavor—avoid bottled if possible.
- Parmesan cheese: Freshly grated is best for flavor and melting.
- Seasonings: Salt, pepper, and rosemary to taste.
How to make sweet potato gratin
- Preheat the oven to 400°F (200°C). Grease a baking dish with butter or olive oil.
- In a small skillet over medium-high heat, melt the butter. Add the finely chopped onion and sauté until translucent, about 2–3 minutes. Add the minced garlic and cook for 30 seconds until fragrant.
- Pour in the heavy whipping cream and stir. In a small bowl, mix the warm cream with arrowroot (or cornstarch) to form a slurry, then return it to the skillet and cook, stirring, until the sauce thickens. Remove from heat and set aside.
- Arrange a single layer of thinly sliced sweet potatoes in the prepared dish, overlapping slightly. Season with salt, pepper, and a sprinkle of rosemary. Pour half of the onion-cream mixture over the potatoes and sprinkle with 1/2 cup of grated Parmesan.
- Add a second overlapping layer of sweet potato slices. Season again, pour the remaining cream mixture over the top, and sprinkle with the remaining Parmesan.
- Bake for 30–40 minutes, until the sweet potatoes are tender when pierced with a fork. Allow the gratin to rest 5–10 minutes before serving so the sauce can thicken slightly.
What is a gratin?
A gratin is a classic French preparation: a creamy, baked dish often topped with cheese and sometimes breadcrumbs. Potatoes are a common base, but other vegetables work well too.
Can you make it ahead of time?
Yes. You can assemble the dish a day in advance and keep it covered in the refrigerator. Bake it when you’re ready to serve. Alternatively, bake the gratin up to 3–4 days ahead and store it in the fridge; reheat covered with foil at 350°F until warmed through.
Can you freeze it?
Yes. Freeze the gratin baked or unbaked. Wrap the dish securely in foil and place it in a freezer bag. Thaw overnight in the fridge before baking or reheating.
Recipe Notes and tips
- Slice the sweet potatoes uniformly so they cook evenly; a mandoline speeds this step.
- If you want a more browned top, briefly broil the gratin after baking—watch closely to avoid burning.
- The sweet potatoes should be fork-tender when done.
- To make this vegetarian, choose Parmesan labeled as free from animal rennet.
More Sweet Potato Recipes
- Sweet Potato Fries with Paprika Garlic Aioli
- Baked Sweet Potato Slices
- Mashed Sweet Potatoes with Bacon
- Sweet Potato Sausage Frittata
- Baked Sweet Potato Chips
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Sweet Potato Gratin
Ingredients
- 1.5 lbs sweet potatoes, about 2–3, thinly sliced (⅛”)
- 3 Tablespoon butter
- ½ cup onion, finely chopped
- 3–4 garlic cloves, minced
- 1 cup heavy whipping cream
- 1 Tablespoon arrowroot starch, (or cornstarch)
- Juice of ½ lemon
- 1 cup parmesan cheese, thinly grated
- Salt and pepper, to taste
- 1 teaspoon rosemary
Instructions
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Preheat the oven to 400°F (200°C) and grease a baking dish.
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Sauté onion in butter until translucent, add garlic for 30 seconds, then stir in cream. Whisk a slurry of cream and arrowroot, return to the skillet, and cook until the sauce thickens. Remove from heat.
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Layer thin sweet potato slices in the dish, season with salt, pepper, and rosemary. Pour half the cream mixture over the layer and sprinkle with ½ cup Parmesan.
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Add a second layer of potatoes, season, pour the remaining sauce, and top with the remaining Parmesan.
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Bake 30–40 minutes until potatoes are tender. Let rest 5–10 minutes before serving.
Notes
- Even slices ensure even cooking; use a mandoline for consistency and speed.
- Broil briefly if you prefer a more browned top—watch carefully.
- Check doneness with a fork; potatoes should be tender.
- Use vegetarian Parmesan if avoiding animal rennet.
Nutrition
Nutrition Disclaimer: Nutritional values are estimates and may vary with ingredients, brands, and portion sizes.
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