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