Combine olive oil, lime juice, onions, garlic, saffron, salt and pepper in a large resealable freezer bag. Place lamb pieces in bag; seal and shake to combine. Marinate lamb, refrigerated, overnight or up to 24 hours.
Preheat a barbecue for medium-high heat and lightly oil the cooking 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 grilled marked, about 5 minutes on each side. Sprinkle with sumac, if desired, before serving.
Sumac comes from dried berries that are grown in the Mediterranean. They taste sour and fruity. You can find them in Middle Eastern speciality shops or online.