Crack the eggs into a bowl. Whisk until light and frothy.
Heat a non-stick frying pan over medium heat and add the oil. Add the mushroom and sauté for 2 minutes until coloured. add the butter and season well. Toss around to coat the foaming butter in the mushrooms. Remove from the heat and add to the bowl with the eggs.
Pour in the eggs mixture back in the hot pan and move the back of a fork through them in small circles, allowing the uncooked egg to flow into the gaps for 1 -2 minutes. Stop stirring once the eggs have begun to set but are still very moist in appearance.
Season with salt and pepper if desired. Remove from the heat and use a spatula to fold the omelette in thirds
Leave to settle for a few seconds, then transfer to a serving plate and serve immediately.