My favourite addition to omelettes is egg yolks :) Just kidding but the yolks are essential to the great flavour. The whites are high protein but low in flavour so you could add some bacon, goat cheese, sundried tomatoes... something like that. With regular omelettes or fritattas, I always add sauteed onions, garlic and wilted greens. You can add any other vegetables though... peppers and mushrooms are good options. And since an egg white omelette will be lacking in fats, I would serve it with some chopped avocado.
I like to add onions, a little garlic, chopped peppers, tomato with whatever veggies i want. Use a bit of cheese to bind the egg whites together and once the beaten egg whites ( whipped till the soft peak stage, no more after that) hit the pan ( should have a little oil on it.) don't touch them! I found the taste of egg whites to be a little bland at first. Maybe try adding one egg yolk to it first, then just pure egg whites. GL! xx
I am a big fan of adding mushrooms, red sweet peppers, wilted spinich, and a little bit of feta cheese (which adds a LOT of flavor). Make sure to use a non stick pan.
One more question. What quantity of egg whites do you use? I used about 2.5 eggs worth. Did the trick, but was a bit thin.