Omelette with Mushrooms, Onion and Tomato

1 First of all, peel the onion, chop the onion, tomato and the mushrooms as thinly as you prefere. Put some olive oil in the frying pan and when is hot add the chopped mushrooms, tomato and onion to it. Fry them normally but be careful they don't turn brown at all.
2 Break and beat the eggs in a large bowl, add the fried stuff (without olive oil) to the bowl and mix everything together, add the salt and black pepper. Add just a tablespoon of olive oil to the frying pan and when hot add the whole mix to it. The heat should be very low because we don't want to burn the base.
3 Wait 10-15 minutes until there is almost no liquid egg on the surface, turn it upside down using a plate (bigger than the frying pan). Now just let the omelette cook for another 5 minutes and it's ready for eat.