Lightly brown the leg of lamb and rub it with a little vinegar, salt, pepper and thyme.Peel the onions and cut them in thick pieces. Place them in a roasting pan and turn them in a mixture of beer, apple vinegar, apple juice, thyme, salt and pepper.
Place the leg of lamb on the onions and cover with aluminium foil. Heat the oven to 130°C. Put the roasting pan in the oven and roast the leg of lamb for 10 hours. Remove the meat and let it cool for 10 minutes before carving and serving it with the baked onions.