1/4 cup salted butter, plus more for baking dish
1/4 cup all-purpose flour
3 1/2 cups half-and-half
1/2 teaspoon garlic powder
8 ounces sharp white Cheddar cheese, shredded (about 2 cups), divided
4 ounces pre-shredded Parmesan cheese (about 1 cup), divided
4 pounds russet potatoes (about 6 large potatoes), peeled and thinly sliced
2 teaspoons kosher salt
1/2 teaspoon black pepper
2 tablespoons chopped fresh chives
Preheat oven to 375°F. Heat butter in a medium saucepan over medium, whisking often, until melted, about 2 minutes. Add flour and cook, whisking constantly, until bubbling, about 1 minute. Slowly whisk in half-and-half. Increase heat to medium-high, and cook, whisking often, until slightly thickened, 5 to 6 minutes. Whisk in garlic powder, 1½ cups of the white Cheddar, and ¾ cup of the Parmesan until cheese is melted and sauce is smooth, about 1 minute. Remove from heat and set aside.
Lightly grease a 13- x 9-inch baking dish with butter. Add one layer of potatoes, shingle-style. Sprinkle with a small amount of salt and pepper. Spread about 1 cup of sauce mixture evenly over potatoes. Repeat layers 4 to 5 more times with remaining potatoes, salt, pepper, and sauce mixture, ending with a layer of sauce on top. Sprinkle top with remaining ½ cup Cheddar and 1/4 cup Parmesan. Lightly grease a piece of aluminum foil with butter and place over baking dish. Bake until potatoes are somewhat tender and liquid is bubbling, about 40 minutes. Remove foil and continue to bake until potatoes are tender and top of the casserole is a deep golden brown, 30 to 35 minutes.
Remove from oven and let stand at room temperature 10 minutes. Top with chives.