In a skillet over medium heat, cook bell pepper and onion until softened, about 4 - 5 minutes. Add spinach and cook until wilted.
In the same pan, add the pre-cooked sausage crumbles and cook until slightly brown.
In a large bowl, whisk together: eggs, egg whites, milk, cheese, salt and pepper.
To the egg and milk mixture, add the veggie and sausage mixture and stir to combine.
Spray a pie dish or casserole dish with non-stick spray. Pour egg and sausage mixture into the pan.
Bake at 350°F for 35–45 minutes, or until the center is set.
Let rest for 10–15 minutes before slicing.