Combine olive oil, lime juice, onions, garlic, saffron, salt and pepper in a large, zip top food storage bag. Place lamb pieces in bag; seal and shake to combine. Marinate lamb, refrigerated, overnight or up to 24 hours.
Preheat barbecue for medium-high heat and lightly oil the grate.
Thread marinated lamb on long thin metal skewers. Thread whole tomatoes on another skewer. Brush lamb and tomatoes with marinade; discard remaining marinade.
Grill kebabs until lamb is cooked to your liking and tomatoes are hot and grill marked, about 5 minutes on each side. Sprinkle with sumac if desired before serving.