In a bowl, mix the eggs with a fork and season with salt and pepper to taste. Heat the butter in a small non-stick frying pan over a medium–high heat until bubbling. Pour in the beaten egg and gently stir with a fork, being careful not to scratch the pan or over-stir the mixture. Once it starts to set, stop stirring and cook until pale golden on the bottom. Place the cheese and mushrooms in the middle of the omelette, then tilt the pan and, using a rubber spatula, gently push the omelette down to the bottom of the pan and fold the lowest part back onto itself, then fold again into a cigar shape. Gently turn the omelette onto a plate to serve (if the omelette isn’t looking too pretty, use a paper towel to reshape it). Drizzle with a good-quality balsamic vinegar (Saporoso or Pukara Estate) to garnish, if wished.