Preheat oven: Heat oven to 375°F (190°C) and grease a 9x13 baking dish.
Cook sausage: In a skillet over medium heat, cook sausage until browned. Drain any excess grease and set aside.
Layer ingredients: Spread the hashbrowns evenly across the greased baking dish. Add cooked sausage, followed by onions and bell peppers if using. Sprinkle shredded cheese on top.
Mix egg mixture: In a bowl, whisk together eggs, milk, salt, and pepper. Pour this mixture evenly over the casserole.
Bake: Place casserole in the oven and bake for 35–40 minutes, or until the eggs are set and the top is golden brown.
Serve: Let it cool for a few minutes before slicing. Serve warm and enjoy!