Heat 2 tbsp olive oil in a non stick frying pan, add the mushrooms and garlic and cook over a high heat for 3-4mins or until golden. Season to taste then transfer to a plate and keep warm. Wipe out the pan.
Beat the eggs with seasoning and 4 tsp cold water. Heat a little of the remaining oil in the frying pan, add a quarter of the eggs and cook for 2-3mins, pushing the cooked egg into the centre of the pan as it cooks, allowing the runny egg to fill its place.
When the base is golden, scatter a quarter of the mushrooms and cheese over the centre of the omelette. Tip the pan to fold the omelette onto a warm plate. Repeat to make 3 more omelettes in the same way.