Preheat the oven to 350 F. In a large pan, cook the Italian sausage with the chopped onion until there is no pink left in the sausage.

Using a large bowl, mix together the beaten eggs, milk, 2 cups of shredded cheese, bell peppers, and hash brown potatoes. Add the cooked sausage and season with salt and pepper

Pour the egg mixture into a 9 x 13-inch casserole dish and bake for 45 to 50 minutes or until the egg is set in the middle. Add the remaining cup of shredded cheese on top and bake for 10 more minutes or until the cheese is melted and bubbly