Veal kidneys – Rognons de veau

A knob of butter is melted in a frying pan, and pieces of veal kidney are fried on each side for 5 minutes with salt and pepper. The frying pan is then warmed with port or sherry vinegar. Certain bistros serve it so; others add mustard or fresh cream. They are then covered with this sauce.
But specialists will tell you that kidneys should be eaten nature, without sauce.