1 large bag hashbrowns
8 tablespoons melted butter
1/2 c. chopped onion
1 12 oz bag chopped Cheddar cheese
1 8oz container sour cream
1 can cream of mushroom soup

In a 9 x 13 inch casserole, combine all ingredients. Sprinkle additional Cheddar cheese over the top. Refrigerate overnight. Bake at 350°F for 45 minutes.