Crack the eggs into a mixing bowl with a pinch of salt and pepper. Whisk the eggs with a fork until smooth and frothy.
Melt the butter in a frying pan, over medium heat. Add the mushrooms and onion to the pan, stirring until tender, and then pour in the egg mixture, surrounding the veggies.
When the omelet begins to cook and get firm, but still has a little raw egg on top, sprinkle cheese over the egg.
Using a spatula, carefully ease around the edges of the omelet, and then fold it over in half. When it starts to turn golden brown underneath, remove the pan from the heat and slide the omelet on to a plate.
