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.