Preheat Panini maker or George Foreman grill. This melt can also be cooked in a nonstick pan. See step #5 below.
Pour 2 egg whites into a custard cup or small bowl to create a round shape. Sprinkle in onions. Cover with a microwave-safe plate and cook in microwave for 1-2 minutes until cook through.
Spread the outside of the sandwich thin, both top and bottom, with ½ teaspoon of butter on each. Split open and spread the inside of the bottom slice with pizza sauce. On top of sauce place cooked eggs and top with 1 slice of cheese. Place remaining top piece of sandwich thin, butter side out.
To cook in Panini maker or George Foreman grill: Place in maker and cook for 3-4 minutes until melted and golden brown. Remove with a spatula as the cheese oozes out a bit while cooking. Remove any melted pieces on the Panini maker and add to plate. I love it that way!
To cook in pan on stove top: Be sure to coat a nonstick pan with cooking spray. Cook over medium heat for about 2-3 minutes until golden, turn over, and continue to cook for another 2-3 minutes until cheese is melted and the sandwich thin is golden brown.
Serve with a fork and knife.