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Sweet potato gratin transforms ordinary sweet potatoes into a rich, cheesy, and comforting dish perfect for holiday meals or any cozy dinner. Layers of vibrant sweet potato slices, creamy herb-infused sauce, and melted Gruyère create an irresistible casserole that brings warmth and flavor to the table. This recipe is a great way to surprise family with a twist on traditional au gratin potatoes and highlight the natural sweetness of the tubers.
I first made this dish one Thanksgiving when trying to impress my family with something different. Now it is one of our most requested dishes because it balances savory and sweet perfectly with every bite melting in your mouth.
Ingredients
- Unsalted butter: softened used for greasing the baking dish and building flavor in the sauce choose fresh butter without added salt
- Garlic: chopped adds fragrant depth fresh cloves are best for a pungent and clean taste
- Fresh thyme leaves: bring earthiness and aroma opt for vibrant, green sprigs
- Fresh sage leaves: chopped adds a subtle peppery note complementing sweet potatoes perfectly
- Kosher salt: season carefully helps bring out all flavors
- Heavy cream: creates luscious richness look for full-fat cream for best results
- Whole milk: lightens the sauce without diluting flavor use fresh milk
- Ground cinnamon: adds warmth and a hint of sweetness choose fresh and aromatic cinnamon powder
- Ground nutmeg: provides subtle spice freshly ground if possible for brightest flavor
- Large eggs: help thicken and bind the custard
- Grated Parmesan cheese: adds sharpness and savory umami look for quality hard cheese preferably freshly grated
- Freshly ground black pepper: seasons the sauce with mild heat
- Sweet potatoes: peeled and sliced thin mix of orange, purple, and white adds color and varied sweetness choose firm, smooth-skinned potatoes without blemishes
- Shredded Gruyère cheese: melts beautifully with a nutty flavor freshly shredded cheese works best
Step-by-Step Instructions
- Sauté the Aromatics:
- Melt two tablespoons of butter in a small saucepan over medium heat until foaming stops. Add chopped garlic and stir for about 30 seconds until fragrant. Stir in chopped thyme, sage, and half a teaspoon of salt, cooking for another 30 seconds to release herb flavors.
- Simmer the Cream Mixture:
- Add heavy cream, whole milk, cinnamon, and nutmeg to the saucepan. Bring to a gentle simmer without boiling and cook for 5 minutes while stirring occasionally until slightly thickened. Remove from heat once ready.
- Temper and Combine Eggs:
- Beat eggs in a medium bowl. Gradually whisk one cup of the warm cream mixture into the eggs a few tablespoons at a time to prevent scrambling. Return the egg mixture to the saucepan with the remaining cream and whisk well. Incorporate half a cup of Parmesan cheese and season with freshly ground pepper to taste.
- Layer the Potatoes and Cheese:
- Butter a casserole dish and arrange a layer of thin sweet potato slices with some overlap. Lightly season with salt. Sprinkle two-thirds cup of shredded Gruyère cheese over the potatoes, then pour two-thirds cup of the cream mixture evenly on top. Repeat layering three more times, ending with sauce on the final layer. Finish by sprinkling the remaining Parmesan cheese over the top.
- Bake Covered Then Uncovered:
- Butter a piece of foil and cover the dish tightly. Bake in a preheated 375-degree oven for 30 minutes. Remove the foil and continue baking for about 30 minutes until the edges are bubbling and the top is crisp and golden. Garnish with fresh thyme before serving.
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My favorite ingredient has to be the fresh sage leaves. Their subtle sharpness and aroma balance the rich creaminess beautifully. I remember one winter dinner when my grandmother was surprised to find such a rich, cozy dish made from sweet potatoes which she usually only thought of as plain baked or mashed. Since then, it always earns compliments.
Storage Tips
To keep leftovers fresh, store the gratin in an airtight container in the refrigerator for up to four days. Reheat in a 375-degree oven for about 20 minutes until warm throughout to retain its creamy texture. For longer storage, freeze portions in freezer-safe containers or wrap the entire dish tightly in plastic and foil for up to three months. Thaw overnight in the refrigerator before reheating.
Ingredient Substitutions
If Gruyère is unavailable, Swiss cheese or a mild fontina can be good alternatives with similar meltability and flavor profiles. Milk can be replaced with a lighter cream option if you want a richer sauce. Fresh herbs like rosemary can substitute for thyme but use sparingly as it is strong. Non-dairy milk such as cashew or oat milk could work, but the sauce texture will be less rich.
Serving Suggestions
Sweet potato gratin pairs wonderfully with roasted turkey or chicken, making it an excellent side for special occasions. It also complements winter greens like sautéed spinach or kale for a balanced meal. Add a crisp green salad with vinaigrette to cut through the rich creaminess and brighten the plate.
Cultural and Historical Context
Gratin dishes have origins in French cuisine where starchy vegetables or potatoes are baked in a creamy, cheesy sauce to form a golden crust. Sweet potatoes empower this tradition with natural sweetness and vibrant color, bridging classic technique with hearty American holiday flavors. This dish brings a familiar European comfort food into seasonal celebrations with a twist.
Common Recipe Questions
- → What type of sweet potatoes work best?
Any variety works well, but mixing orange, purple, and white sweet potatoes adds color and depth to the layers.
- → How should potatoes be sliced?
Slice potatoes thinly and evenly, about 1/8 inch thick. A mandoline makes this easier, but a sharp knife and patience also work.
- → Can it be prepared ahead of time?
Yes, you can assemble and bake in advance, then refrigerate up to 3 days. Alternatively, assemble unbaked and refrigerate for up to 2 days before baking.
- → What cheeses complement the dish?
Gruyère adds nuttiness and meltiness, while Parmesan provides sharp, savory notes. Using both creates a balanced flavor and texture.
- → How should leftovers be stored and reheated?
Store leftovers in a sealed container in the fridge for 3–4 days or freeze up to 3 months. Reheat at 375°F for about 20 minutes until warmed through.
- → What spices enhance the cream sauce?
Cinnamon and nutmeg add subtle warmth, while fresh thyme and sage impart herbal brightness, perfectly complementing the sweet potatoes.