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Home › appetizers › Sweet Potato Gratin

Sweet Potato Gratin

medium👥 6American, Frenchsweet potatogratinholiday

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⏱

30m

Prep

🔥

60m

Cook

⏰

90m

Total

👥

Servings

📊

medium

Difficulty

About this recipe

Sweet potato gratin represents the perfect marriage of French culinary technique with America's beloved autumn vegetable. This luxurious side dish elevates the humble sweet potato from simple roasted fare to restaurant-worthy elegance, layering paper-thin slices with a silky, herb-infused cream sauce that's both sophisticated and deeply comforting. The dish traces its roots to the classic French gratin dauphinoise, but this version celebrates the natural sweetness and vibrant colors of sweet potatoes while balancing them with savory herbs and nutty cheeses. What makes this particular version exceptional is the thoughtful blend of aromatic spices and fresh herbs that complement rather than compete with the sweet potatoes' inherent flavor. The combination of warming cinnamon and nutmeg with earthy thyme and sage creates a complex flavor profile that feels both familiar and surprising. Using a mix of purple, orange, and white sweet potato varieties not only creates a stunning visual presentation but also adds subtle flavor variations throughout each layer. The dual cheese topping of sharp Gruyère and nutty Parmesan provides the perfect golden, bubbling finish that makes this gratin irresistible.

Why you'll love this

✓ **Stunning presentation** with layers of colorful sweet potato varieties creating a natural rainbow effect that's Instagram-worthy
✓ **Complex flavor balance** where warming spices meet fresh herbs and creamy cheese for a sophisticated taste profile
✓ **Make-ahead friendly** since it can be assembled hours in advance and baked when needed, perfect for entertaining
✓ **Comfort food elegance** that transforms simple ingredients into a restaurant-quality side dish worthy of special occasions

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Ingredients

For

servings

3Tbsp. unsalted butter, softened, divided (plus more for foil)
4tsp. cloves garlic, chopped
2Tbsp. chopped fresh thyme leaves (plus more for serving)
1Tbsp. chopped fresh sage leaves
0.5tsp. kosher salt ((plus more for seasoning potatoes))
1cup heavycream
0.75cup wholemilk
0.25tsp. ground cinnamon
0.25tsp. ground nutmeg
2largeeggs
0.75cup gratedParmesan, divided ((about 2 oz.))
Freshly ground black pepper
2lb. sweet potatoes, peeled and sliced 1/8" thick ((mix of purple, orange, and white))
2.67cups shreddedGruyère cheese, divided ((about 8 1/2 oz.))

Step-by-step instructions

Preheat 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.

💡 Pro tip: Keep the cream mixture at a gentle simmer, never boiling, to prevent curdling and ensure silky texture

In 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.

💡 Pro tip: Temper the eggs slowly by adding hot cream gradually while whisking constantly - this prevents scrambled eggs in your sauce

Line 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.

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Butter 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.

💡 Pro tip: The foil prevents over-browning while ensuring the potatoes cook through completely before the final golden finish

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Chef's tips

heavy cream

Don't use half-and-half as a substitute; the fat content is crucial for proper thickening and richness.

sweet potatoes

Use a mix of varieties for visual appeal - orange for sweetness, purple for color, white for subtle flavor. Slice uniformly using a mandoline for even cooking.

Gruyère cheese

Don't substitute with regular Swiss - Gruyère's nutty, complex flavor is essential. Freshly shred for best melting quality.

fresh thyme and sage

Fresh herbs are non-negotiable here - dried versions will muddy the delicate cream sauce.

Variations & substitutions

VeganReplace butter with vegan butter, use full-fat coconut milk instead of cream and milk, substitute nutritional yeast and cashew cream for cheeses, and use flax eggs
Gluten-freeThis recipe is naturally gluten-free as written - just ensure your spices and cheese haven't been cross-contaminated during processing
Less sweetOmit the cinnamon and nutmeg, increase the sage, and consider mixing in some regular Yukon potatoes with the sweet potatoes
UpgradeAdd truffle oil to the cream mixture, use aged Gruyère, finish with crispy fried sage leaves and toasted hazelnuts for extra luxury

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Storage & shelf life

🧊

fridge

4 days

Cover tightly and reheat covered at 350°F until warmed through, about 20-25 minutes

❄️

freezer

2 months

Wrap tightly in plastic and foil. Thaw overnight in fridge before reheating - texture may be slightly different

🌡️

room temp

2 hours

Only safe for short periods during serving - cover with foil to retain heat

🍽️ How to serve

Serve this gratin piping hot directly from the baking dish, allowing the bubbling edges to showcase the beautiful golden-brown top. Cut into generous squares and plate with a fresh sprinkle of thyme leaves for color and aroma. This rich, creamy side pairs beautifully with roasted meats, especially turkey, pork tenderloin, or herb-crusted beef, and makes an elegant centerpiece for holiday tables.