Combine the olive oil, garlic, lemon juice and spices in a bowl and add the cubed lamb shoulder. Coat the lamb well, cover and leave to marinade for as long as you can, ideally overnight, but at least for four hours.
To assemble the skewers alternate the cubed, marinated lamb (3-4 pieces per skewer) with red bell pepper and half of a quartered onion (about 3 layers). Arrange on a platter until ready to cook, brushing any remaining marinade over the skewers.
BBQ for 12-16 minutes, turning frequently until cooked to the desired doneness. Leave the skewers to rest for five minutes before eating.
Serve with flatbread, salad and plain natural yogurt, if desired.