3 pounds russet potatoes, peeled and cubed
8 ounces sour cream (or Greek yogurt)
1/2 cup whole or 2% milk, plus more as needed
8 ounces sharp cheddar cheese, grated (about 2 1/2 cups)
2 large shallots, minced (about 1/2 cup)
2 teaspoons kosher salt
Freshly ground black pepper, to taste
1/4 cup grated Parmesan cheese
Preheat oven to 350°F. Add potatoes to a large stock pot, cover with water, and bring to a boil. Cook until the potatoes are cooked through and a knife pierces easily to the center. Drain and return the potatoes to the pot. Add the sour cream and milk to the pot and, using a potato masher, smash until desired texture is reached. (For light and creamy potatoes you can use an electric hand mixer.) Fold in the cheddar cheese, shallots, salt and pepper. Taste and adjust seasonings.
Transfer the potato mixture to a medium gratin or baking dish (approximately 2 1/2 quart). Bake for 30 minutes, or until warmed through. Sprinkle with Parmesan cheese and finish under the broiler until golden, about 2 minutes.
This casserole can be made up to 24 hours in advance. Bring to room temperature for at least 30 minutes before cooking.