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.