Prepare the Hashbrowns. Ensure the hashbrowns are thawed and pat them dry with a paper towel to remove excess moisture.
Mix the Ingredients. In a large bowl, combine the hashbrowns, sour cream, cream of mushroom soup, melted butter, shredded cheddar cheese, chopped onion, salt, and pepper. Mix until well combined.
Layer and Top. Transfer the mixture to a greased baking dish, spreading it evenly. Top with additional shredded cheese.
Bake to Perfection. Bake in a preheated oven at 350°F for about 45-50 minutes, or until the casserole is bubbly, and the top is golden brown.