Preheat your oven to 375°F (190°C). Grease a 9x13 inch baking dish with cooking spray or butter. Thaw the frozen hash browns slightly if they're completely frozen solid - this helps them cook more evenly.

In a large skillet over medium heat, cook the breakfast sausage until browned and cooked through, breaking it up into small pieces as it cooks. Remove the sausage with a slotted spoon and set aside. In the same skillet, sauté the diced onion and bell pepper in the remaining sausage drippings (or add 1 tablespoon olive oil) until softened, about 5 minutes.

Spread the hash browns evenly in the prepared baking dish. Top with the cooked sausage and sautéed vegetables. Sprinkle 1½ cups of shredded cheese over the mixture. In a medium bowl, whisk together the eggs, milk, salt, pepper, and garlic powder until well combined. Pour the egg mixture evenly over the casserole.

Sprinkle the remaining ½ cup of cheese over the top. Bake for 45-55 minutes, or until the eggs are set and the top is golden brown. The center should not jiggle when you shake the pan. Let the casserole rest for 5-10 minutes before slicing and serving.