Preheat Oven: Preheat your oven to 350°F (175°C).
Cook Beef and Onions: In a large skillet, cook the ground beef over medium heat until browned. Add the chopped onion and cook until softened. Drain any excess fat.
Mix Ingredients: In a large bowl, combine the cooked beef and onions with the cream of mushroom soup, sour cream, and half of the shredded cheese. Stir until well mixed.
Add Hashbrowns: Fold in the thawed hashbrowns until evenly distributed.
Assemble Casserole: Transfer the mixture into a greased 9x13-inch baking dish. Sprinkle the remaining cheese on top.
Bake: Bake in the preheated oven for 45 minutes, or until the cheese is bubbly and the top is golden brown.
Serve: Let the casserole cool slightly before serving. Enjoy with a fresh salad or garlic bread on the side.