Sweet Potato Gratin

This Sweet Potato Gratin takes comfort food to an entirely new level. Imagine layers of tender sweet potatoes baked in a creamy, herb-infused sauce, finished with bubbling Gruyère and Parmesan cheese. It’s elegant enough for the holidays yet simple enough for a cozy weeknight dinner. The combination of sweet, savory, and nutty flavors makes this gratin unforgettable — a dish that looks as beautiful as it tastes.

Table of Contents

Table of Contents

Why You’ll Love This Recipe

• Creamy & Decadent: A luscious blend of cream, milk, and melted cheese creates a velvety sauce that coats every slice.
• Herb-Infused Flavor: Fresh thyme, sage, and garlic bring aromatic warmth that perfectly complements the natural sweetness of the potatoes.
• Perfect for Any Occasion: Serve it alongside roasted meats for Thanksgiving or as a comforting vegetarian main.
• Visually Stunning: Using a mix of orange, purple, and white sweet potatoes creates a beautiful layered presentation.
• Crowd-Pleaser: This dish impresses every time — it’s classic, cozy, and irresistibly cheesy.

Ingredients Needed

For the Creamy Herb Sauce

• 3 tablespoons unsalted butter, divided (plus extra for greasing foil and dish)
• 4 cloves garlic, finely chopped
• 2 tablespoons fresh thyme leaves, chopped (plus more for garnish)
• 1 tablespoon fresh sage leaves, chopped
• 1 cup heavy cream
• ¾ cup whole milk
• ¼ teaspoon ground cinnamon
• ¼ teaspoon ground nutmeg
• Kosher salt and freshly ground black pepper, to taste

For the Egg-Cheese Mixture

• 2 large eggs, room temperature
• ¾ cup grated Parmesan cheese (about 2 oz.), divided

For Layering

• 2 pounds sweet potatoes (a mix of purple, orange, and white), peeled and thinly sliced (about ⅛ inch thick)
• 2⅔ cups shredded Gruyère cheese (about 8½ oz.), divided

How to Make Sweet Potato Gratin

Step 1: Prepare the Dish and Infuse the Cream

Preheat your oven to 375°F (190°C). Butter a 2-quart casserole or gratin dish with 1 tablespoon butter.
In a small saucepan over medium heat, melt the remaining 2 tablespoons butter. Once it stops foaming, add the chopped garlic and sauté for about 30 seconds, just until fragrant — this builds a flavorful base.
Stir in the thyme, sage, and ½ teaspoon salt; cook for another 30 seconds to release their oils.
Add the cream, milk, cinnamon, and nutmeg. Bring to a gentle simmer (do not boil) and cook until slightly thickened, about 5 minutes. Remove from heat.

Step 2: Temper the Eggs

In a medium bowl, lightly beat the eggs. Slowly pour about 1 cup of the warm cream mixture into the eggs, whisking constantly — this gradual process prevents scrambling.
Then whisk the tempered egg mixture back into the saucepan. Add ½ cup Parmesan cheese, season with black pepper, and stir until smooth.

Step 3: Assemble the Layers

Arrange a thin layer of sweet potato slices in the prepared dish, slightly overlapping each piece. Sprinkle lightly with salt.
Top with about ⅔ cup Gruyère cheese and ⅔ cup of the cream mixture.
Repeat this layering process three more times, finishing with the cream mixture. Sprinkle the remaining ¼ cup Parmesan over the top for a golden crust.

Step 4: Bake Until Golden

Butter a piece of foil and cover the dish tightly, buttered side down — this prevents sticking.
Bake for 30 minutes covered, then remove the foil and continue baking for another 30 minutes, or until the edges are bubbling and the top is beautifully browned.
Garnish with fresh thyme leaves before serving.

Serving and Storage Tips

• To Serve: Let the gratin rest for 10–15 minutes after baking — this helps the layers set and makes slicing easier. Serve warm as a side dish with roast chicken, turkey, or beef.
• Storage: Cover leftovers tightly and refrigerate for up to 4 days.
• Reheating: Warm in a 350°F (175°C) oven for 15–20 minutes, or until heated through. Add a splash of milk or cream if the gratin looks dry.
• Freezing: Not recommended, as the creamy sauce may separate upon thawing.

Helpful Notes

• Use a Mandoline: For even, paper-thin slices that cook uniformly.
• Mix of Sweet Potatoes: Combining different colors adds visual appeal and subtle flavor variation.
• Cheese Substitutions: Try Comté or Fontina for a different flavor profile.
• Make-Ahead Tip: Assemble up to a day in advance, cover, and refrigerate. Bring to room temperature before baking.
• Extra Golden Top: For a crispier finish, broil for 2–3 minutes at the end — watch closely!

Conclusion

This Sweet Potato Gratin is everything you want in a comforting side dish — creamy, cheesy, beautifully layered, and bursting with herbaceous flavor. Whether it’s gracing your holiday table or brightening up a simple family dinner, it never fails to impress.

If you try this recipe, please leave a star rating and comment below — I’d love to hear how it turned out!
Don’t forget to share it on Pinterest or Facebook to inspire your friends and family.
👉 What’s your favorite cheese to use in a gratin? Let us know in the comments!

Frequently Asked Questions (FAQ)

Can I make this Sweet Potato Gratin ahead of time?

Yes! Assemble the dish up to 24 hours ahead, cover tightly, and refrigerate. Let it come to room temperature before baking.

Can I use regular potatoes instead of sweet potatoes?

You can — Yukon Golds or russets work well. The flavor will be less sweet but still creamy and delicious.

How do I make this gluten-free?

This recipe is naturally gluten-free since it contains no flour or breadcrumbs. Just double-check that your cheese and cream are certified gluten-free.

Why did my sauce curdle?

Curdling can happen if the cream mixture gets too hot or the eggs are added too quickly. Always temper the eggs gradually and avoid boiling the sauce.

Can I freeze Sweet Potato Gratin?

It’s best enjoyed fresh. Freezing can cause the creamy sauce to separate, affecting texture. If needed, refrigerate and reheat within a few days for best results.