I hear your family couldn't decide between mashed potatoes and potatoes au gratin? My mashed potato gratin combines both in one baking dish, so everyone at the table gets their favorite. The bottom layer is creamy sour cream mashed potatoes with green onions, while the top is made with thinly sliced potatoes and bubbly Gruyère cheese.
This post may include affiliate links. Please read my disclosure policy.
This is THE potato side dish recipe for families with strong opinions about their potatoes. And let's face it, Thanksgiving dinner is not the time to start negotiating over side dishes. Two potato dishes in one. You get the creamy comfort of mashed potatoes and the golden, cheesy topping of a classic potato gratin without making two separate sides.
I originally tested this recipe with a more basic mashed potato layer, but the two layers needed more than just a difference in texture. The sour cream and onions provide the flavor contrast that makes the whole dish work.
📖Why My Recipe Works
- Contrast in flavor and texture. Sour cream and green onions give the bottom layer a tangy, savory flavor that stands up to the richer Gruyere topping.
- Simple ingredients, impressive presentation. Overlapping potato slices create a beautiful top layer, making this a great side dish for Thanksgiving, Christmas, or Sunday dinner.
🧾Ingredient Notes
- Potatoes - I used russets for their high starch content but Yukon Golds work too. You'll need 5 pounds total, divided between the two layers.
- Sour Cream - Full-fat sour cream adds tanginess and body to the mashed potato layer. Greek yogurt can work as a substitute, although the flavor will be slightly different.
- Green Onions - These add a mild onion flavor without overpowering the potatoes. Chives are a good alternative.
- Gruyere Cheese - A great melting cheese with a slightly nutty flavor. Swiss cheese is a more affordable substitute.
- Parmesan Cheese - Freshly grated Parmesan is my preference over the shelf-stable variety.
⏲️How to Make Cheesy Mashed Potato Gratin
- Start your potatoes in cold, salted water so they cook evenly. You want them very soft, not just fork-tender.
- After mashing with warm milk and butter, fold in the sour cream and green onions by hand, then spread the mixture evenly into your buttered baking dish.
- Toss the slices with melted butter, Parmesan, and seasoning until evenly coated.
- Arrange them in overlapping rows over the mashed potatoes. Keep the slices fairly flat rather than stacking them too deeply so they cook through in the oven.
- Top with cheese before baking for 20 to 30 minutes. The cheese should be melted and the sliced potatoes tender.
- I like to finish the dish under the broiler for a few minutes to get the cheese extra bubbly. Keep a close eye on it because Gruyere can go from golden to burnt quickly.
🥗Serving Suggestions
This recipe is definitely a little more work, but it is show-stopping. Which makes it a perfect pairing with main courses like my sous vide prime rib, BBQ grilled turkey breast, or my popular sage roasted Thanksgiving turkey.
Because this gratin recipe is so rich, I like to serve it with a crisper vegetable like sauteed Spanish green beans, garlic broccolini or a roasted beet and apple salad.
👩🏻🍳Jen's Testing Notes
- After draining, let the potatoes sit in the colander for a couple of minutes. This allows excess moisture to evaporate so your mashed potatoes won't turn watery beneath the gratin layer.
- Warm the milk and butter together before mixing them into the potatoes. Cold milk can cool the potatoes too quickly and make them harder to mash evenly.
- Avoid overmixing the mashed potatoes, which can make potatoes gluey.
- A mandoline makes it easier to get consistent slices, which is important because unevenly sliced potatoes won't cook at the same rate.
- Let the casserole rest for about 10 minutes before serving. This gives the layers a chance to settle and makes scooping easier.
Did you enjoy making this recipe? Show it some love with ⭐⭐⭐⭐⭐ and leave a comment! Hungry for more FLAVORFUL recipes?
📖 Recipe Card
Mashed Potato Gratin
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.3 pounds russet potatoes, 2 Tablespoons kosher salt
- 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!1 cup whole milk, 4 Tablespoons unsalted butter
- 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.¾ cup sour cream, 4 green onions
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.2 pounds russet potatoes, 4 Tablespoons unsalted butter, ½ teaspoon dried thyme, ½ teaspoon kosher salt, ¼ teaspoon black pepper, ¼ teaspoon ground nutmeg, ½ cup parmesan cheese, 1½ cups 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.
Comments
No Comments