Melt the butter in a small non-stick frying pan. Add the sliced mushrooms and cook over a high heat until golden. Remove from pan with a slotted spoon. Stir a handful of chopped watercress into the beaten eggs and season with salt and pepper. Pour the eggs into the hot pan and tilt the pan to cover the base with the mixture.

Reduce to a moderate heat and cook until the omelette is just set and the underside is golden brown. Scatter the mushrooms and Stilton over the top. Slide the omelette onto a plate and fold in half. Garnish with extra watercress and serve immediately.