Description
This Sweet Potatoes Au Gratin recipe is a rich, cheesy baked casserole featuring thinly sliced sweet potatoes smothered in a creamy Gruyere and cheddar cheese sauce, enhanced with aromatic herbs and a crispy breadcrumb topping. Perfect as a comforting side dish, it blends the natural sweetness of sweet potatoes with savory, nutty flavors in a warm, golden gratin.
Ingredients
Scale
Sweet Potatoes
- 3 lbs. sweet potatoes (cut into ⅛” slices)
- 2 tsp. salt (divided)
- ½ tsp. black pepper
Cheese Sauce
- 2 Tbsp. butter (salted)
- 2 Tbsp. flour (all purpose or gluten-free 1-to-1 blend)
- 4 cloves garlic (finely minced)
- 1 ½ cups milk (regular or plant-based, room temperature)
- 1 cup Gruyere Cheese (shredded)
- 1 cup cheddar Cheese (shredded)
- 1 tsp. fresh thyme
- 1 tsp. fresh rosemary
- ¼ tsp. nutmeg (ground)
- 1 tsp. salt (remaining)
Crumb Topping
- 1 Tbsp. butter (melted)
- ¼ cup Panko crumbs (or regular breadcrumbs)
- ¼ cup grated Parmesan cheese
Instructions
- Preheat the Oven: Preheat your oven to 375 degrees Fahrenheit to get it ready for baking the gratin.
- Prepare the Potatoes: Rinse and scrub the sweet potatoes thoroughly to remove any dirt. Dry them completely using a dish towel. Carefully slice the sweet potatoes into very thin ⅛-inch slices using a mandoline slicer or a sharp chef’s knife to ensure even cooking.
- Layer Potatoes in Baking Dish: Spray a 13×9-inch baking dish with non-stick cooking spray. Arrange half of the sliced sweet potatoes evenly on the bottom of the dish. Sprinkle with ½ teaspoon salt and ¼ teaspoon black pepper. Add the remaining sweet potatoes on top, then season again with ½ teaspoon salt and ¼ teaspoon black pepper for balanced seasoning.
- Make the Cheese Sauce: In a medium saucepan over medium heat, melt 2 tablespoons of butter. Add the minced garlic and sauté for 30 to 60 seconds until fragrant but not browned. Lower the heat to medium-low and sprinkle in the flour. Whisk continuously to form a smooth paste (roux). Gradually add the room temperature milk in ¼ cup increments, whisking well after each addition until the sauce thickens and is smooth.
- Add Cheese and Herbs: Remove the saucepan from the heat. Stir in the shredded Gruyere and cheddar cheeses until fully melted. Then whisk in fresh thyme, rosemary, ground nutmeg, and the remaining teaspoon of salt to infuse rich flavors into the sauce.
- Assemble and Bake Covered: Pour the cheese sauce evenly over the layered sweet potatoes in the baking dish. Use a spatula to spread the sauce so all slices are coated. Cover the dish tightly with aluminum foil and bake in the preheated oven for 30 minutes.
- Prepare and Add Crumb Topping: While the gratin bakes, combine the melted butter, Panko crumbs, and grated Parmesan cheese in a bowl. After 30 minutes, remove the foil from the dish and sprinkle this crumb topping evenly over the potatoes.
- Bake Until Browned and Tender: Return the uncovered dish to the oven and bake for an additional 15 to 20 minutes. This allows the topping to brown nicely and the sweet potatoes to become fork tender.
- Serve: Remove from the oven and serve immediately. Garnish with a sprinkle of finely chopped fresh rosemary and thyme for a fresh herbal aroma and extra flavor.
Notes
- For uniform cooking, slice sweet potatoes very thin (about ⅛ inch thick).
- Use plant-based milk to make this recipe dairy-free, but ensure your cheeses fit your dietary preferences.
- Panko breadcrumbs add extra crunch but regular breadcrumbs work perfectly as well.
- Covering the gratin during the first baking prevents the sauce from drying out.
- This dish can be prepared a day ahead; refrigerate covered and bake as directed before serving.
