Marinate the meat as far in advance as possible. Mix the beer, onion, juniper berries, oregano and bay leaves. Place the meat in the marinade.
Light a BBQ grill, with one hot zone and one slightly cooler zone. It is important that you have enough space for grilling over indirect heat, because the lamb needs to be grilled for a long time.
Place the meat on the grill until it is slightly brown on both sides. Then turn it in the marinade again. Continue taking it in turns to grill the meat and then placing it in the marinade in order to obtain several layers of caramelized marinade on the outside of the crust of the meat.
The lamb needs to spend about one and a half hours on the grill, so you will need to add more coals as time goes by.
Allow the meat to stand before serving. Serve with creamed green cabbage and roasted root vegetables.