Mashed Potato Gratin
This mashed potato gratin combines creamy sour cream mashed potatoes with a potato gratin potato topping. A golden, bubbly potato casserole that's perfect for holiday dinners and family gatherings.
Servings: 8
Calories: 515kcal
Ingredients
Mashed Potato Layer
- 3 pounds russet potatoes peeled and cut into 1 inch pieces
- 2 Tablespoons kosher salt divided
- 1 cup whole milk
- 4 Tablespoons unsalted butter
- ¾ cup sour cream
- 4 green onions thinly sliced
Potatoes au Gratin Layer
- 2 pounds russet potatoes peeled and cut into ⅛ inch slices
- 4 Tablespoons unsalted butter melted
- ½ teaspoon dried thyme
- ½ teaspoon kosher salt
- ¼ teaspoon black pepper
- ¼ teaspoon ground nutmeg
- ½ cup parmesan cheese
- 1½ cups gruyere cheese
Instructions
- Preheat the oven to 425℉. Butter a 13x9-inch baking dish.
Mashed Potato Gratin
- Put the potatoes in a large pot with 1 ½ tablespoons salt and cover with cold water. Bring to a boil over high heat, then reduce to a rolling boil and cook for about 20 minutes, until the potatoes fall apart when poked with a fork.
- Drain the potatoes into a colander. Let them sit for 2 to 3 minutes to drain off excess moisture.
- While the potatoes are cooking, heat the milk and butter in a small saucepan over low heat. Do not boil!
- Transfer the potatoes to a mixing bowl and roughly mash them with a potato masher. Mix the potatoes with an electric mixer, then slowly pour in half the milk mixture. Continue mixing until the potatoes are thick and creamy with very few lumps, adding the warm milk as needed. Season to taste with the remaining salt, then stir in the sour cream and onions by hand. Spread evenly in your prepared baking dish.
Potatoes au Gratin Layer
- Add the sliced potatoes, butter, seasonings, and Parmesan to a bowl and toss until evenly coated. Arrange the potatoes on top of the mashed potatoes in flat, overlapping rows (see photos in post). Top with the Gruyere cheese.
- Bake for 20 to 30 minutes, until the top is brown and the potatoes are cooked through. Cool for 10 minutes before serving.
Notes
Store leftover mashed potato gratin in an airtight container in the refrigerator for up to 4 days, and reheat covered in a 350°F oven until hot throughout.
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