Cheesy Potato Casserole

Name: Cheesy Potato Casserole
Ingredients: ¼ cup salted butter, plus more for baking dish; ¼ cup all-purpose flour; 3½ cups half-and-half; ½ teaspoon garlic powder; 8 ounces sharp white Cheddar cheese, shredded (about 2 cups), divided; 4 ounces shredded Parmesan cheese (about 1 cup), divided; 4 pounds russet potatoes (about 6 large potatoes), peeled and thinly sliced; 2 teaspoons kosher salt; ½ teaspoon black pepper; 2 tablespoons chopped fresh chives