Preheat the oven to 350°F (175°C).
In a large skillet over medium heat, cook the breakfast sausage until browned and fully cooked. Drain excess grease and set aside.
In a large mixing bowl, whisk together the eggs, milk, salt, pepper, garlic powder, and onion powder until well combined.
In a greased 9x13 inch baking dish, layer the thawed hash browns evenly across the bottom.
Spread the cooked sausage over the hash browns, followed by the optional bell peppers and onions if using.
Pour the egg mixture over the sausage and hash browns, ensuring everything is evenly coated.
Sprinkle the shredded cheddar cheese on top.
Bake in the preheated oven for 45-50 minutes, or until the casserole is set and the top is golden brown.
Let cool for 5-10 minutes before slicing. Garnish with fresh parsley if desired.