Place the lamb in a bowl with the garlic, spices, mint and 1 tablespoon of the oil, then rub really well to coat the meat in the spices. Marinate in the refrigerator for at least 15 minutes.
Thread the lamb onto 4 long metal skewers, alternating with the onion, green pepper and tomato wedges. Arrange on the rack of a foil-lined grill tray, drizzle with the remaining oil and cook under a hot grill for 7–10 minutes, turning occasionally, until everything is lightly charred and the lamb is cooked to your liking.
Serve the kebab skewers with bowls of steamed couscous, and garlic sauce or tzatziki, if desired.