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This sweet potato gratin transforms ordinary sweet potatoes into a rich and cheesy dish perfect for holiday dinners or cozy winter meals. Layers of tender sweet potatoes soaked in a creamy herb-spiced sauce and topped with bubbling Gruyère cheese make this gratin an irresistible comfort food upgrade. It’s a fantastic way to bring variety and color to your table, especially when you use a mix of orange, purple, and white sweet potatoes.
I first made this for a Thanksgiving where I wanted to surprise my family with something unexpected. The combination of the herby cream and nutmeg-infused potatoes won everyone over instantly, and it has become a tradition since.
Ingredients
- Unsalted butter: softened to help brown the top and enrich the sauce
- Garlic: chopped finely to release fragrance and deepen flavor
- Fresh thyme leaves: packed with aromatic and slightly minty notes
- Fresh sage leaves: earthy and slightly peppery, adding complexity
- Kosher salt: essential for seasoning the potatoes and cream sauce perfectly
- Heavy cream: creates the rich, silky base for this gratin
- Whole milk: used to lighten the cream for a balanced texture
- Ground cinnamon: offers a subtle warmth that complements the sweet potatoes
- Ground nutmeg: adds fragrant spice rounding out the sauce flavors
- Large eggs: bind the cream sauce and give the gratin structure
- Grated Parmesan: brings saltiness and sharpness in the layers enhancing the cheese blend
- Freshly ground black pepper: for bright peppery notes and seasoning
- Sweet potatoes: peeled and sliced paper-thin for even cooking, mixing varieties brings color and diverse flavors
- Shredded Gruyère cheese: melts beautifully giving a nutty and buttery richness
Step-by-Step Instructions
- Sauté the Aromatics:
- Melt butter over medium heat in a small saucepan. Once foaming stops, add garlic and cook just until fragrant about 30 seconds to avoid bitterness. Add thyme, sage, and salt, stirring occasionally for half a minute. Mix in heavy cream, milk, cinnamon, and nutmeg. Bring the sauce to a gentle simmer without boiling, stirring often until it thickens slightly, about five minutes. Remove from heat.
- Temper the Eggs:
- In a separate bowl, beat the eggs lightly. Slowly pour one cup of the warm cream sauce mixture into the eggs in a steady stream while whisking constantly. This gradual warming prevents the eggs from scrambling. Pour the tempered egg mixture back into the saucepan with the remaining cream sauce, whisk until smooth. Stir in half a cup of Parmesan cheese and season with freshly ground black pepper.
- Layer the Gratin:
- Grease the baking dish with butter. Arrange one layer of thin sweet potato slices on the bottom, overlapping slightly to cover the base. Sprinkle lightly with salt. Add two thirds cup shredded Gruyère cheese, then spoon over two thirds cup of the cream mixture. Repeat layering three times in total, finishing with cream sauce on top. Sprinkle remaining Parmesan on the surface.
- Bake and Finish:
- Butter a piece of foil and cover the gratin dish tightly. Bake in a preheated oven at 375 degrees Fahrenheit for 30 minutes. Remove the foil and continue baking until the edges bubble and the surface turns golden brown about 30 minutes more. Garnish with fresh thyme leaves before serving.
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One of my favorite parts is using a mix of sweet potato colors which makes each slice visually stunning. Watching the bubbling gratin come out of the oven and seeing those golden edges is pure joy and always feels like a celebration at the dinner table.
Storage Tips
Store leftovers in an airtight container in the refrigerator and reheat in the oven at 375 degrees Fahrenheit for about 20 minutes. The gratin also freezes well for up to three months—perfect for batch cooking holiday meals.
Ingredient Substitutions
If you cannot find Gruyère, Emmental or Fontina cheeses make good alternatives for a similar melt and flavor profile. For a lighter version, swap heavy cream for half and half but expect a slightly less rich sauce. Fresh herbs can be swapped for dried, but use about one-third of the fresh quantity.
Serving Suggestions
Pair this gratin with roasted meats such as turkey or pork, or serve alongside a green salad for a lighter complement. It also shines next to slow-cooked stews or simple roasted winter vegetables.
Pro Tips
- Take your time sautéing the garlic and herbs to build a deep flavor foundation; rushing this step results in a less aromatic sauce.
- Use a mandoline or very sharp knife to slice potatoes evenly for uniform cooking.
- Butter the foil to prevent it from sticking and ruining the gratin’s golden crust.
Commonly Asked Questions
- → What sweet potato types work best for gratin?
Any variety is great. Mixing orange, purple, and white sweet potatoes creates vibrant layers and varied flavors.
- → How should I slice the potatoes evenly?
A mandoline ensures uniform 1/8" slices for even baking, but a sharp knife and patience also work well.
- → Can I prepare the dish ahead of time?
Yes, you can assemble and bake in advance, then refrigerate for up to 3 days. Alternatively, assemble raw and refrigerate up to 2 days before baking.
- → What cheeses complement the sweet potatoes?
Gruyère adds a nutty richness while Parmesan brings a sharp, savory contrast that enhances the cream sauce.
- → How do I store leftovers properly?
Store cooled portions in a sealed container in the fridge for 3-4 days or freeze for up to 3 months. Reheat in the oven until warmed through.
- → What spices add depth to this gratin?
A touch of ground cinnamon and nutmeg combined with fresh thyme and sage create a warm, herbaceous flavor profile.