Soak 8 wooden skewers in cold water for at least 30 minutes, to prevent them burning while the kebabs are cooking; if using metal skewers, you can leave out this step, of course.
Mix together the yogurt, garlic, mint and mint sauce in a bowl. Add the lamb and stir well. Cover and leave to marinate in a cool place for 10 minutes or preferably in the refrigerator for at least 1 hour, to allow the meat to absorb more of the flavours.
Thread the lamb and onion and green pepper wedges alternately on to the wooden or metal skewers (metal skewers help the meat to cook right through), arrange on a grill rack and cook under a preheated medium-hot grill for 8–10 minutes, turning once, until the meat is cooked through and the onions and peppers are softened and starting to brown. Alternatively, grill the skewers in a preheated griddle pan or over a barbecue.
Serve hot with green salad, couscous and lemon wedges, if liked.