Mom's Party Potatoes

1 (2 pound) package frozen cubed hash brown potatoes

thawed

2 cups shredded mozzarella cheese

1 (16 ounce) container sour cream

1 (10.75 ounce) cancream of chicken soup

½ cup melted butter

½ cup chopped onion

1 teaspoon salt

½ teaspoon ground black pepper