Finely dice sweet peppers and sweet onions, and chop parsley.
Cook the sausage on medium-high heat just until there’s no pink. Add the diced onions and peppers and sauté until fully cooked. Turn off the heat, stir in the parsley, and let cool.
Whisk 6 eggs, parmesan, half-and-half, salt, and pepper until smooth and thin.
Grease the sheet pan with butter. Add about half of the cooked mix-ins generously, saving the rest for another batch. Pour the egg mixture over evenly, top with Italian cheese blend, and scatter parsley on top.
Bake at 365°F for 20–25 minutes. Around the 10-minute mark, before the eggs fully set, pull the pan out and stir with a fork or spatula. Continue baking until set.