Cheesy Hash Brown Casserole

Name: Cheesy Hash Brown Casserole
Ingredients: 6 Tbsp. unsalted butter, plus more for pan; 1 small onion, finely chopped or coarsely grated; ¼ cup all-purpose flour; 2½ cups whole milk; 1 cup sour cream; 2 tsp. Diamond Crystal or 1¼ tsp. Morton kosher salt; ½ tsp. freshly ground black pepper; ¼ tsp. garlic powder; 1 30-oz. package frozen shredded hash browns, thawed; 10 oz. sharp cheddar, coarsely grated (about 2½ cups), divided