Have the beef cut from between the third and fourth ribs and cut it into 12 very thin entrecôtes - or ask the butcher to do it for you. Each one should weigh about 75 g (2½ oz) after trimming. Season the entrecôtes lightly.
Heat 20 g (¾oz) butter in a sauté pan and soften the onions for 5 minutes, without letting them brown and stirring with a wooden spoon. Deg