Preheat your oven to 350°F and lightly grease a 9x13 inch baking dish.
Spread the thawed hash browns evenly in the bottom of the prepared baking dish.
Layer the cooked, crumbled sausage over the hash browns.
Sprinkle the diced onions and bell peppers over the sausage layer.
Top with shredded cheddar cheese, distributing it evenly across the casserole.
In a mixing bowl, whisk together the eggs, milk, salt, black pepper, and garlic powder until well combined.
Pour the egg mixture over the cheese layer, making sure to cover the ingredients evenly.
Bake in the preheated oven for 45 minutes, or until the casserole is set and the top is golden brown.
Allow the casserole to cool for a few minutes before slicing into squares and serving.