Preheat the oven to 350 degrees Fahrenheit. Spray a casserole dish with the cooking spray.
Cook the penne pasta according to the directions on the package then drain.
Melt the margarine in a nonstick saucepan. Sauté the onions for 4 minutes over medium heat. Whisk the flour and 1/2 cup of milk together in a bowl then add to the onions. Cook to mix, stirring frequently. Pour in the rest of the milk and cook for approximately 10 minutes.
Turn the heat off and stir in the cheese, mustard, 1/4 teaspoon of paprika, salt, and pepper. Mix together 1/2 cup of the cheese sauce and the egg substitute in a bowl. Then, combine the egg and cheese mixture with the rest of the cheese sauce.
Mix the cheese combo in with the pasta. Put the mixture in the casserole dish. Put the bread crumbs in a bowl and stir in the parsley. Sprinkle the bread crumbs on top of the pasta. Gently spray the bread crumbs with the cooking spray and top with 1/4 teaspoon of paprika.
Bake for approximately 25 minutes or until the top is lightly browned and the sauce bubbles.