Heat the olive oil in a small non-stick frying pan. Tip in the mushrooms and fry over a high heat, stirring occasionally for 2-3 mins until golden. Lift out of the pan into a bowl and mix with the cheese and parsley.
Place the pan back on the heat and swirl the eggs into it. Cook for 1 min or until set to your liking, swirling with a fork now and again.
Spoon the mushroom mix over one half of the omelette. Using a spatula or palette knife, flip the omelette over to cover the mushrooms. Cook for a few moments more, lift onto a plate and serve.