Heat a large frying pan over medium-high heat. Add the remaining
butter and allow it to melt. Add the beaten eggs. Use the
flat side of a fork to stir the eggs until they start to set. Smooth the top and leave the eggs to
cook. Arrange the cooked mushrooms on the center of the eggs. Fold the side of the omelet up to form it into a triangle. Transfer to a heated serving plate, sprinkle lightly with
lemon juice, and serve.