In small non stick frying pan melt the butter and add the mushrooms. Cook for three minutes until soft.
Add the egg and season. Cook on a low heat for 4-5 minutes until the egg is cooked.
Turn out onto a plate upside down.
To make the topping, melt the butter in a small pan, add the mushrooms and cook for four minutes until soft and golden.
Serve on top of the omelette.