Rub the meat in salt and pepper and a pinch of oregano or marjoram and oil. Take so much oil that the meat floats a little. Add the onion rings. Let rest for at least 12 hours.
Make a beech wood fire under a swivel grill - a tripod with a round grille. When embers have formed, place the meat on the wire rack and stir calmly over the fire for at least 40 minutes. At the same time, move the grille further down. Place the pickled onions in an aluminum dish on the grill about 20 minutes before the end of the grill.
The meat is ready when it is very dark on the outside.