Egg White Omelette With Onions, Mushrooms, Smoked Salmon ,Tomato

Egg White Omelette with fresh mushrooms, onions, smoked salmon and tomato

1 T. olive oil or butter
1/3 c. onion,
1/3 c. button mushrooms,
1/2 c. fresh diced tomato,
2 T. diced smoked salmon,