This make-ahead cheesy mashed potato casserole upgrades the classic with sour cream, scallions, and a golden, crispy top for an easy, flavorful holiday side.
If baking immediately, preheat the oven to 375°F. Place the potatoes in a large pot and cover with cool water by about 2 inches. Salt generously and bring to a boil. Boil until the potatoes are fork-tender, 7 to 10 minutes. Drain the potatoes and return them to the pot, along with the grated cheddar and 2 tablespoons of the butter.
Mash the potatoes with a potato masher until the butter and cheese are melted and the potatoes are mostly smooth (some lumps are okay). Stir in the sour cream and green onions. Taste and adjust the seasoning with salt.
Grease a 13 x 9-inch baking dish with nonstick cooking spray or butter. Spread the potatoes evenly in the prepared dish. Cut the remaining 1 tablespoon of butter into small pieces. Top the potatoes with the grated parmesan and dot with the pieces of butter. If you are preparing the casserole for later, cover tightly with plastic wrap and refrigerate for up to 2 days.
Bake the casserole until the top is lightly golden brown in spots, 40 to 45 minutes. If desired, turn on the broiler and broil until browned to your liking, 1 to 2 minutes more. If the casserole was refrigerated, remove it from the refrigerator 1 to 2 hours before baking. Bake at 375°F until the top is lightly golden brown in spots, 45 to 60 minutes. Refrigerate leftovers in an airtight container for up to 5 days. Reheat individual servings in the microwave. Love the recipe? Leave us stars and a comment below!