Melt a little of the butter in an omelette pan, add the mushrooms and sauté for 5–6 minutes until cooked and any moisture has evaporated. Remove the mushrooms from the pan.
Melt a little more butter in the same pan and add one-quarter of the beaten egg. Season well with pepper and stir with a wooden spoon, bringing the cooked egg to the centre of the pan and allowing the runny egg to flow to the edge of the pan and cook.
When there is only a little liquid egg left, sprinkle over a few mushrooms and some of the parsley and Gruyère. Fold the omelette over, tip on to a warm serving plate and keep warm. Repeat with the remaining ingredients to make 4 omelettes. Serve warm.