1. Preheat oven to 400°F.
2. Add the sausage to a medium pan and cook over medium-high heat until browned, breaking up the sausage in small pieces.
3. Transfer sausage to a large mixing bowl. Keep the sausage grease in the pan.
4. Add the onion, red and yellow pepper to pan, and sauté for about 5 minutes until cooked. Add the garlic and sauté for an additional 2 minutes.
5. Put the vegetable mixture into the mixing bowl with sausage; add diced red potato. Add Cheddar and mozzarella cheese to the mixing bowl with the sausage and veggies. Mix well to combine.
6. In a separate bowl, whisk together the eggs, milk and black pepper. Pour the egg mixture into the veggie and sausage mixture, and stir to combine.
7. Pour the mixture into a greased 9x13-inch baking dish and top with the Parmesan cheese.
8. Cover with aluminum foil and bake for 30 minutes.
9. Remove the aluminum foil and bake for an additional 10-15 minutes until the potatoes are tender.
10. Let the casserole rest for 5 minutes. Sprinkle with green onions or parsley before serving.