If using link sausage, squeeze meat from casing into a 10-inch nonstick skillet. Cook sausage over medium heat until browned all over, about 5 minutes, breaking up meat with tongs and a wooden spoon. Remove to a plate. Add bell pepper to skillet and cook, stirring often, until soft, 3 to 5 minutes.
In a medium bowl, beat eggs, milk, parsley, salt, and pepper until blended. Stir in half of the mozzarella and half of the Parmesan.
Add sausage back to skillet, and pour egg mixture over top. Reduce heat to medium-low and cook for 2 minutes, gently scraping the bottom of the pan to loosen the eggs. Cover and cook until eggs are set on the bottom but the top is still wet, another 6 to 8 minutes.
Meanwhile, preheat the broiler and set the oven rack to the top third of oven. Set bread slices on a baking sheet and broil until golden brown, 1 to 2 minutes per side. Remove.
Scatter remaining cheeses over top of egg mixture and transfer pan to broiler. Broil until top is set, puffed, and lightly browned, about 1 minute. Cut into wedges and garnish with more chopped fresh parsley, if you like. Serve with toasted bread.
Tip: Want more flavor? Sauté your favorite vegetables along with the bell pepper. Mushrooms and zucchini make great additions. Use pre-cut vegetables from the grocery store to save time.