In a small nonstick pan melt the butter over medium heat and add the mushrooms. Cook until soft, about 5 minutes. Season with salt and pepper. Pour the eggs over and keep stirring with a rubber spatula until the bottom begins to set, about 1 minute. Season with salt and pepper. Let the bottom cook another 30 seconds so that it will release from the pan. From one side roll the omelet up by thirds, then place a plate on top of the pan. Flip the pan over with the plate so that the omelet falls cleanly onto the plate. Garnish with chopped parsley or more cooked mushrooms.