Spray A 9X5-Inch Glass Or Microwave-Safe Loaf Pan With Cooking Spray; Sprinkle The Onion, Green Bell Pepper, And Mushrooms Into The Pan, And Toss Lightly With A Fork Just To Mix. Season With Salt And Black Pepper, And Pour In The Egg Whites. Cook In A Microwave Oven On High Setting For 3 Minutes. Remove And Stir The Cooked Egg White From The Side Of The Pan Into The Rest Of The Ingredients; Cook For 3 More Minutes On High. If The Omelet Is Still A Little Runny On Top, Slice It Into Chunks And Turn Them Over In The Loaf Pan; Microwave For 30 More Seconds On High. Adjust Salt And Pepper, And Serve.