Heat a large frying pan over medium-high heat. Add the remaining
butter and allow it to melt and color slightly. Add the beaten eggs. Use the
flat side of the fork to stir the eggs until they start to set. Smooth the top and leave the eggs to
cook. Spoon two-thirds of the cooked
kidney in a
line across the center of the omelet. Fold the omelet into thirds and flip it onto a heated serving plate.