Sweet Potato Gratin is a cozy baked casserole of thinly sliced sweet potatoes tucked into a garlicky cream sauce with Parmesan and a touch of thyme, baked until tender with a burnished, cheesy top. It’s the perfect side for holidays, Sunday dinners, or any night you want something creamy and special.
You’ll see how to slice the potatoes so they cook evenly, build a sauce that bakes up silky (not greasy), and get that gorgeous browned top without overcooking the bottom.
Why You’ll Love This Sweet Potato Gratin
- Ultra-creamy interior – Warm cream + cheese emulsify for a custardy texture.
- Crispy, golden lid – The top browns beautifully while the center stays tender.
- Make-ahead friendly – Assemble ahead and bake later.
- Flexible flavors – Add Gruyère, herbs, or a hint of spice.
- Holiday star – Feels fancy but is simple to make.
Ingredients
For the gratin:
- 2 1/2–3 lb (1.1–1.4 kg) sweet potatoes, peeled and thinly sliced (2–3 mm)
- 1 tablespoon unsalted butter (for greasing the dish)
- 1 cup (240 ml) heavy cream
- 1 cup (240 ml) whole milk (or use all cream for extra richness)
- 2–3 cloves garlic, minced
- 1 teaspoon fresh thyme leaves (or 1/2 tsp dried)
- 3/4 teaspoon kosher salt, plus more to taste
- 1/2 teaspoon black pepper
- Pinch red pepper flakes (optional)
Cheese:
- 1 cup (85 g) finely grated Parmesan (or 1/2 Parmesan + 1/2 Gruyère)
Finishing:
- 1 tablespoon butter, cut in small dots (optional)
- Extra thyme leaves and Parmesan for the top
How to Make Sweet Potato Gratin
1) Heat oven & prep dish
Preheat to 375°F (190°C). Butter a 9×13-inch (or similar) baking dish.
2) Warm the cream mixture
In a small saucepan over low heat, combine cream, milk, garlic, thyme, salt, pepper, and red pepper flakes. Warm gently 3–4 minutes (don’t boil). This blooms the flavors and helps the gratin set silky.
3) Layer the potatoes
Fan half the sliced sweet potatoes into the dish in slightly overlapping rows. Sprinkle with about one-third of the cheese. Repeat with remaining potatoes and another third of the cheese.
4) Pour & top
Pour the warm cream mixture evenly over the potatoes (tilt the pan to distribute). Top with the remaining cheese and dot with the butter if using.
5) Bake
Cover loosely with foil and bake 30 minutes. Remove foil and bake 20–30 minutes longer, until the top is deep golden and a knife slides through the center easily.
Tip: If the top is browning too fast, tent with foil; if it’s pale at the end, broil 1–2 minutes, watching closely.
6) Rest & serve
Let the gratin rest 10–15 minutes to set for cleaner slices. Sprinkle with a little fresh thyme and serve warm.
Serving Suggestions
- Roast chicken, turkey, beef, or glazed ham
- Simple green salad with lemon vinaigrette
- Sautéed green beans or roasted Brussels sprouts
- Cranberry relish or chutney for contrast
Cook’s Tips
- Thin, even slices: Use a mandoline for uniform 2–3 mm slices so everything cooks at the same pace.
- Warm dairy only: Cold cream can split and won’t coat as smoothly.
- Don’t flood the dish: You want the cream to just come up to the top layer; too much liquid = soupy.
- Cheese choice: Parmesan gives savory depth; Gruyère adds luxe melt and nuttiness.
- Resting time: Essential for neat squares and the most velvety texture.
Variations & Add-Ins
- Gruyère & leek: Sauté 1 thinly sliced leek in butter; layer with potatoes and use Gruyère.
- Smoky chipotle: Add 1/2 tsp chipotle powder to the cream for subtle heat.
- Bacon thyme: Scatter crisp bacon bits between layers, reduce added salt slightly.
- Herb-forward: Add chopped rosemary or sage with the thyme.
- Extra rich: Use all cream and a mix of Parmesan + Gruyère.
Storage & Reheating
- Fridge: Cover and refrigerate up to 3 days.
- Reheat: 300°F (150°C) oven, covered, until warmed through; uncover a few minutes to re-crisp the top.
- Freezer: Not ideal—cream-based gratins can separate, but leftovers can be frozen tightly wrapped and reheated low and slow.
FAQ
Can I slice potatoes ahead?
Yes. Keep submerged in cold water up to 24 hours; dry thoroughly before layering.
Can I use half-and-half?
Yes. It’s slightly lighter; sauce may be a touch less silky.
Can I add onions?
Layer very thin onion rings with the potatoes (or use sautéed leeks).
This Sweet Potato Gratin layers tender slices in a garlicky thyme cream with a savory, caramelized cheese top. It’s luxurious yet easy, pairs with nearly any main, and always brings that “wow” moment when you set it on the table.
Best Sweet Potato Gratin – Creamy Garlic Parmesan Layers with Crispy Top
Thin sweet potato slices baked in a silky garlic–thyme cream with Parmesan until tender inside and golden on top. A cozy, elegant side for holidays or weeknights.
Ingredients
- 2 1/2–3 lb (1.1–1.4 kg) sweet potatoes, peeled & thinly sliced (2–3 mm)
- 1 tbsp butter (for greasing dish)
- 1 cup (240 ml) heavy cream
- 1 cup (240 ml) whole milk (or more cream)
- 2–3 cloves garlic, minced
- 1 tsp fresh thyme (or 1/2 tsp dried)
- 3/4 tsp kosher salt, plus more to taste
- 1/2 tsp black pepper
- Pinch red pepper flakes (optional)
- 1 cup (85 g) finely grated Parmesan (or 1/2 Parm + 1/2 Gruyère)
- 1 tbsp butter, in small dots (optional)
- Extra thyme & Parmesan for topping
Instructions
- 1. Heat oven: 375°F (190°C). Butter a 9×13-inch dish.
- 2. Warm dairy: Gently heat cream, milk, garlic, thyme, salt, pepper, and flakes 3–4 minutes (don’t boil).
- 3. Layer: Arrange half the potatoes; sprinkle ~1/3 cheese. Repeat with remaining potatoes and another 1/3 cheese.
- 4. Pour & top: Add warm cream mixture. Top with remaining cheese; dot with butter.
- 5. Bake: Cover with foil 30 min; uncover and bake 20–30 min until tender and deeply golden.
- 6. Rest: Stand 10–15 min before serving. Garnish with thyme and Parmesan.
Notes
Slice evenly for uniform cooking; a mandoline helps.
If the top browns too quickly, tent with foil; if pale at the end, broil briefly.
For extra richness, use all cream and add Gruyère to the cheese blend.
Nutrition
Estimated: ~300–380 calories, ~17–22 g fat, ~32–38 g carbs, ~7–9 g protein.
Actual values vary with cheese choice and dairy ratio.