Preheat your oven to 375°F (190°C). Grease a 9x13-inch baking dish with cooking spray or butter.
In a large skillet over medium heat, cook the breakfast sausage until browned and fully cooked, breaking it apart with a spatula as it cooks. Drain any excess grease and set the sausage aside.
In the same skillet, add the diced red onion and bell peppers. Sauté for about 3-4 minutes until softened. Remove from heat.
In a large bowl, whisk together the eggs, milk, garlic powder, black pepper, and salt until well combined.
In the prepared baking dish, spread the thawed hash browns evenly over the bottom.
Layer the cooked sausage over the hash browns, followed by the sautéed veggies. Pour the egg mixture evenly over all the ingredients.
Top with shredded cheddar cheese, spreading it evenly across the casserole.
Bake in the preheated oven for 40-45 minutes, or until the egg is set and the cheese is golden and bubbly.
Let cool for 5-10 minutes before slicing into squares for serving.
Notes
Garnish each serving with fresh parsley and serve with hot sauce for added flavor.