Prepare marinade: mix olive oil, onions, garlic, saffron, salt and black pepper. Cut lamb into 1 cm thick and 4-5 cm long pieces. Do not remove all the fat as you will need it to melt. Marinate overnight (preferrably 24 hours) in refrigerator. Container should be covered.
Thread lamb on long, thin metal skewers. Thread whole tomatoes separately on another skewer. Brush with marinade. Barbeque for about 5-10 minutes on each side, turning frequently. Kabab Barg can also be made in the oven. Prepare as before, pre-heat the grill to a high temperature, and place just under the grill, again turning frequently. Serve hot with basmati rice or on middle-eastern bread. If served with rice, some sumac may be sprinkled on.