Heat a knob of butter in a frying pan and cook the mushrooms until soft, golden and most of their liquid has evaporated. Add the spring onion, chilli and garlic and cook gently for another 3-4 minutes. Season, then tip onto a plate and cover to keep warm.
Wipe the pan and heat a knob of butter. Season the eggs and pour in. Cook gently, drawing in the sides of the omelette, letting the uncooked egg run into the gaps. When the omelette is almost set, sprinkle over the cheese then spoon over the mushroom mix.
Leave for a couple of minutes until the cheese starts to melt then add the rocket and flip over one half. Slide onto a plate.