Sweet Potato Gratin with Gruyère and Parmesan

Scalloped sweet potatoes in a cast iron skillet, topped with pecans, pine nuts, and fresh herbs.

Sweet Potato Gratin with Gruyère and Parmesan

I bake thin spiral-layered sweet potatoes in garlicky thyme cream with Gruyère and Parmesan until tender, creamy, and golden. Then I top it with toasted pecans, pine nuts, and crispy fried sage. This savory sweet potato gratin is a showstopping Thanksgiving and holiday side dish.

Course Side Dish

Cuisine American, French

Keyword savory sweet potato gratin, sweet potato gratin, thanksgiving side dish

Prep Time 30

Cook Time 1 30

Cool time 15

Total Time 2 15

Servings

Calories 396kcal

Author Julia

Ingredients

Sweet Potato Gratin

1 tablespoon unsalted butter softened, for greasing
3 pounds sweet potatoes orange-fleshed, peeled
1 ⅔ cups heavy cream
4 cloves garlic finely grated
1 tablespoon fresh thyme leaves
¼ teaspoon ground nutmeg
1 ½ teaspoons kosher salt
½ teaspoon black pepper freshly ground
1 cup Gruyère cheese freshly grated
⅓ cup Parmesan cheese freshly grated

Topping

⅓ cup pecans roughly chopped
3 tablespoons pine nuts
2 tablespoons unsalted butter
15 fresh sage leaves
Fresh thyme sprigs for garnish

US Customary - Metric

Instructions

Preheat the oven to 350°F (175°C). Lightly grease an 11-inch round ovenproof baking dish or braiser with the softened butter.
Slice the sweet potatoes: Using a mandoline, slice the peeled sweet potatoes into even ⅛-inch-thick rounds. Try to keep the slices together in small stacks as you work. This makes them easier to arrange.
Arrange the spiral: Starting around the outside edge of the prepared baking dish, arrange the sweet potato slices in tightly overlapping rows, leaning them slightly against one another. Continue working toward the center to create a spiral pattern. Use smaller slices to fill any gaps in the middle.
Make the cream sauce: Add the heavy cream, garlic, thyme, nutmeg, salt, and black pepper to a small saucepan. Warm over low heat for 3–5 minutes, stirring occasionally, until fragrant. Do not allow the cream to boil. Remove from the heat and let cool slightly.
Add the grated Gruyère and Parmesan to the warm cream and stir slowly until evenly distributed. The cheese does not need to melt completely. Do not return the saucepan to the heat.
Slowly pour the cream mixture over and between the arranged sweet potatoes. Stir the mixture occasionally as you pour to keep the cheese evenly distributed. Depending on the depth of your dish and how tightly the potatoes are packed, you may not need all of the sauce. Aim to cover the potatoes without drowning them, so the spiral is still visible once the dish is cooked.
Bake covered: Cover the baking dish with a lid or aluminum foil and bake for 45–50 minutes, until the sweet potatoes are beginning to soften. Carefully remove the cover and check the potatoes with the tip of a sharp knife. If they are still very firm, cover and bake for another 10–15 minutes.
Bake uncovered: Remove the lid or foil and continue baking uncovered for 25–35 minutes, until the sweet potatoes are completely tender, the sauce has thickened, and the exposed edges are lightly golden. If the potatoes are tender but need more color, increase the oven temperature to 375°F (190°C) for the final 5–10 minutes, watching carefully to prevent burning.
Toast the nuts: While the gratin finishes baking, place the chopped pecans in a dry skillet over medium-low heat. Toast for 3–5 minutes, stirring frequently, until fragrant. Transfer to a plate. Add the pine nuts to the skillet and toast for 2–3 minutes, stirring frequently, until lightly golden. Transfer to a separate plate, then wipe out the skillet.
Fry the sage: Add the 2 tablespoons of butter to the skillet over medium heat. Once the butter begins to foam, add the sage leaves. Fry for 20–40 seconds, until crisp, then transfer to a paper towel-lined plate. Reserve the fragrant butter.
Rest and serve: Remove the gratin from the oven and let it rest for 15 minutes. Scatter the toasted pecans, pine nuts, and crispy sage leaves over the top, then garnish with fresh thyme. Drizzle with a little reserved sage butter, if desired, and serve warm.

Notes

Baking Dish: I use an 11-inch (28 cm) round baking dish or braiser. Any similar ovenproof dish works if it's deep enough to hold the cream. A smaller dish needs less sauce, and baking time can change, so always check for tenderness.
Alternative Baking Dish: A 9x13-inch dish works too. Arrange the slices in straight overlapping rows and start checking for tenderness a little earlier.
Use a mandoline for even slices: Even ⅛-inch rounds cook at the same rate. Use the hand guard, since sweet potatoes are firm and can slip.
Cheese: Grate the Gruyère and Parmesan from a block. Pre-shredded cheese doesn't melt as smoothly into the cream.
Warm, don't boil, the cream: Gently warm it with the garlic, thyme and seasonings before pouring it over the potatoes. Boiling cream can separate once the cheese goes in. Low and slow is all you need.
Don't drown the potatoes: The full amount of sauce is right for an 11-inch (28 cm) round dish. Too much sauce bubbles over the spiral and hides it, so you may have some left over with a smaller or tightly packed dish.
Bake until tender: Plan on 1 hour 15 minutes to 1 hour 30 minutes. It's done when a knife slides through the potatoes easily, the sauce is gently bubbling and thick, and the edges are lightly browned.
Potatoes tender but tops still pale? Raise the oven temperature slightly for the last few minutes.
Edges browning too fast but potatoes still firm? Loosely cover the dish and keep baking until tender.
Make the toppings: Toast the nuts over medium-low heat, stirring often so they don't burn, then let them cool. Fry dry sage leaves briefly in butter and remove them as soon as they crisp. You can drizzle a little of the sage butter over the gratin.
Let it rest: Rest the gratin for at least 15 minutes so the sauce settles and the spiral shows. This gives the bubbling cream time to settle and thicken slightly, so the potatoes are easier to serve. Resting also helps the spiral show if the sauce rose around the potatoes during baking.
Add the nuts, sage, and thyme right before serving. This keeps them crunchy and stops them from burning in the oven.
Special Tools: 11-inch Round Braiser or Baking Dish | Mandoline | Small Saucepan | Skillet | Sharp Knife | Aluminum Foil | Microplane Grater

Make Ahead

Bake it a day ahead: Bake the gratin, let it cool and refrigerate it without the toppings.
Reheat: Cover and reheat at 325°F (165°C) until heated through, then uncover briefly to refresh the top.
Toppings: Make the toasted nuts and crispy sage separately and add them just before serving.
For the prettiest look, I recommend baking the gratin fresh on the day you serve it.

Storage and Reheating

Refrigerator: Cool the gratin, then move leftovers to an airtight container or cover the dish tightly. Refrigerate within 2 hours of cooking and store for up to 3 to 4 days. Store leftover nuts and crispy sage separately so they don't get soft.
Reheating in the oven: Preheat the oven to 325°F (165°C). Place the gratin in an ovenproof dish, cover loosely with foil and reheat for about 15 to 25 minutes, until heated through (165°F / 74°C in the center). Larger portions need extra time. Remove the foil for the last few minutes to refresh the golden edges.
Reheating in the microwave: This works for single portions, though the potatoes will lose some of their crisp edges.
Freezer: I don't recommend freezing. Cream-based sauces can separate when thawed, and the sweet potatoes may turn softer and release extra liquid.

Nutrition