1. Preheat the Oven: Start by preheating your oven to 350°F.

2. Grease the Baking Dish: Lightly grease a 9×13-inch baking dish to prevent the casserole from sticking.

3. Layer the Hashbrowns: Spread the thawed hashbrowns evenly across the bottom of the prepared baking dish.

4. Add the Sausage: Sprinkle the cooked and crumbled breakfast sausage over the hashbrown layer.

5. Combine the Cheeses: In a bowl, mix the cheddar and mozzarella cheeses, then sprinkle half of the mixture over the sausage layer.

6. Prepare the Egg Mixture: In a separate bowl, whisk together the eggs, milk, salt, pepper, and garlic powder until well combined.

7. Pour the Egg Mixture: Carefully pour the egg mixture over the layers in the baking dish, ensuring even coverage.

8. Top with Remaining Cheese: Finish by sprinkling the remaining cheese mixture over the top of the casserole.

9. Optional Vegetables: If desired, sprinkle the diced bell peppers and onions over the cheese layer.

10. Bake to Perfection: Place the casserole in the preheated oven and bake for 45-50 minutes, or until the center is set and the edges are bubbly.

11. 11. Cool and Serve: Allow the casserole to cool for a few minutes before slicing and serving. Garnish with fresh chopped parsley, if desired.