Chop the onions and roasted red peppers and set aside.
Slice the mushrooms and pop in the microwave on high for approximately 45 sec. You are looking for them to be tender.
Add a pinch of black pepper to the egg whites and beat them with a fork.
Heat the skillet.
I keep a stick of butter in the frig that I used for greasing the skillet. Peel the paper back. Holding the end with the paper, quickly rub it over the surface of the skillet. Weighing the stick of butter before greasing the skillet and immediately after greasing the skillet indicates that 4 g of butter were used in my hands.
Add the egg whites to the hot, greased skillet. Cover the skillet with a lid and let the egg whites begin to cook. After a few minutes, remove the lid. Using a spatula, gently lift the edges of the set egg whites and let the egg whites on top run underneath the set egg whites. Put the lid back on and continue to cook.
When the top of the egg whites looks dry, spread the onions on the top half of the egg whites. Spread the mushrooms on top of the onions followed by the roasted red pepper and the crumbled feta cheese.
Fold the bottom half of the egg whites over the fillings. Gently press down and cook another 30 sec or so.
Gently slide the omelette out of the skillet and onto a plate. Cut the omelette in half and serve.
Notes
6 whole eggs can be used in place of the egg whites.