Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13 inch baking dish.
In a large skillet, brown the sausage over medium-high heat. Drain off any excess grease. Add the chopped onion and green bell pepper and cook until softened, about 5-7 minutes.
Stir in the cream of mushroom soup and sour cream until well combined.
Fold in the thawed hash brown potatoes and half of the shredded cheddar cheese.
Pour the mixture into the prepared baking dish.
Sprinkle the remaining cheddar cheese over the top.
Bake for 45-50 minutes, or until bubbly and the cheese is melted and lightly browned.
Let stand for a few minutes before serving.
Notes
This casserole is great for a crowd and can be made ahead of time and reheated.