Steak With Onions

1 kg beef rib steak, 5 cm thick
6 to 8 tablespoons olive oil
3 large onions, thinly sliced
3 tablespoons red wine vinegar or Dijon-type mustard
Salt and pepper
15 to 20 cl beef or veal stock

Heat 4 tablespoons of the oil in a heavy saute pan and sear the steak on both sides over high heat, turning it several times. After about 5 minutes, when the steak is well colored, turn the heat to low and cook for 15 to 20 minutes, or until done. Remove the steak to a warm serving dish and keep it hot.

Discard the fat from the pan, add the remaining oil and heat it. Add the onions and fry them until they are golden brown. Then stir in the vinegar or mustard, salt and pepper and the stock. Cook for a minute, then pour the mixture over the steak and serve.