Sweet Potato Gratin

Name: Sweet Potato Gratin
Prep time: 30 min
Total time: 2 h
Yield: 12 serving(s)
Ingredients: 3 tbsp. unsalted butter, softened, divided, plus more for foil; 4 tsp. cloves garlic, chopped; 2 tbsp. chopped fresh thyme leaves, plus more for serving; 1 tbsp. chopped fresh sage leaves; Kosher salt; 1 c. heavy cream; 3/4 c. whole milk; 1/4 tsp. ground cinnamon; 1/4 tsp. ground nutmeg; 2 large eggs; 3/4 c. grated Parmesan (about 2 oz.), divided; Freshly ground black pepper; 2 lb. sweet potatoes (mix of purple, orange, and white), peeled and sliced 1/8" thick; 2 2/3 c. shredded Gruyère cheese (about 8 1/2 oz.), divided

By Brooke CaisonUpdated: Nov 9, 2023

Approved by the Delish Test Kitchen

4.6

9 Ratings

tri color sweet potato gratin with gruyere and thyme

PHOTO: LUCY SCHAEFFER; FOOD STYLING: BROOKE CAISON

Yields:

12 serving(s)

Prep Time:

30 mins

Total Time:

2 hrs

Cal/Serv:

286

Jump to recipe

Your sweet potatoes are tired of being baked. Give them the chance to become something extra special this holiday by turning them into sweet potato gratin. Just like the OG potatoes au gratin, these sweet potatoes are layered with plenty of cheese and the signature herby cream sauce before being baked until golden brown and bubbly. Add this to your Thanksgiving dinner spread (Grandma will never see it coming), or serve with any of our fall or winter dinners for a cozy, hearty addition. Here's how to make it the best it can be:

What kind of sweet potatoes are best?
Any kind of sweet potato! This is where you get to have a little fun. Sweet potatoes (not to be confused with yams) come in all shapes, sizes, and colors. Use a beautiful mix of orange, purple, and white sweet potatoes to create colorful layers, or, mix and match with your two favorite colors. You could stick with all orange, but where’s the fun in that?

How to slice the potatoes:
Using a mandoline is certainly the easiest way to get uniform, 1/8" potato slices, but if you don’t have one, a sharp knife and patience will also do the trick. Just be sure to peel the potatoes before slicing.

Tip: No need to waste the end pieces of the potato! Hide the smaller end pieces of the potato in the bottom and middle of the casserole and save the uniform pieces for the top layer.

Make it ahead:
If you want to make this ahead, assemble and bake the gratin, then let cool completely. Wrap the entire pan with plastic wrap and refrigerate for up to 3 days. If you want to make this ahead without baking, you can also layer everything into the casserole dish and cover it tightly with plastic wrap. Store the dish in the refrigerator for up to 2 days.

Storage:
Any leftovers will keep in a resealable storage container for 3 to 4 days or in the freezer for up to 3 months. Reheat at 375° for about 20 minutes or until warmed through.

Have you made this yet? Let us know how it went in the comments below!

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Ingredients

3 Tbsp.
unsalted butter, softened, divided, plus more for foil
4 tsp.
cloves garlic, chopped
2 Tbsp.
chopped fresh thyme leaves, plus more for serving
1 Tbsp.
chopped fresh sage leaves
Kosher salt
1 cup
heavy cream
3/4 cup
whole milk
1/4 tsp.
ground cinnamon
1/4 tsp.
ground nutmeg
2
large eggs
3/4 cup
grated Parmesan (about 2 oz.), divided
Freshly ground black pepper
2 lb.
sweet potatoes (mix of purple, orange, and white), peeled and sliced 1/8" thick
2 2/3 cups
shredded Gruyère cheese (about 8 1/2 oz.), divided

Directions

1. Step 1Preheat oven to 375°. Butter a 2-quart casserole or gratin dish with 1 tablespoon butter. In a small saucepan over medium heat, melt 2 tablespoons butter. When butter stops foaming, add garlic and cook, stirring, until fragrant, about 30 seconds. Add thyme, sage, and 1/2 teaspoon salt and cook, stirring occasionally, until heated through, about 30 seconds. Add cream, milk, cinnamon, and nutmeg. Bring to a simmer (do not let boil) and cook, stirring occasionally, until slightly thickened, about 5 minutes. Remove from heat.
2. Step 2In a medium bowl, beat eggs. Transfer 1 cup cream mixture to a liquid measuring cup or bowl with a spout. Whisking constantly, slowly stream cream mixture into eggs a few tablespoons at a time until incorporated. Add egg mixture to saucepan with remaining cream mixture and whisk until combined. Whisk in 1/2 cup Parmesan; season with pepper.
3. Step 3Line bottom of prepared dish with a layer of potatoes, slightly overlapping slices; season with a pinch of salt. Top with 2/3 cup Gruyère and 2/3 cup cream mixture. Repeat 3 times, ending with cream mixture. Top final layer with remaining 1/4 cup Parmesan.
4. Step 4Butter a piece of foil and tightly cover dish. Bake gratin 30 minutes, uncover, and continue to bake until sides are bubbling and golden brown, about 30 minutes more. Top with thyme.