Remove silver membranes from the fillets, place tips on the heads and tie into a uniformly thick piece. Rub the meat with oil and pepper. Cut the onion into strips, as well as the bell pepper.
Heat the rest of the oil and brown the meat in it. Cover and fry over reduced heat, turning every now and then. Add the onion and pepper strips after 10 minutes. Salt, de-twine and keep the fillet warm. Add the remaining ingredients to the stock, cook briefly, season to taste. Serve hot.