In a non-stick frying pan over medium low heat melt the 1/2 the butter and quickly add sliced mushroom and fry 5 minutes. Once done transfer to a plate and set aside.
In the same pan melt remaining butter and fry onion until gets fragrant, about 5 minutes. Meanwhile beat the eggs and add in dry ingredients plus vinegar and make it ready to pour over fried onion once it's done.
Once onion is done pour egg mixture over it and quickly cover until it's half cooked. Uncover frying pan and quickly spread cheese over and then fried mushrooms, again cover till cheese is melted.
Turn the flame off and garnish with chopped cress. Cut it into slices like a pizza and serve .