Soak the saffron for 10 minutes. Grate the onions and drain the excess juice. 
Combine grated onions with ground beef in a large bowl. Add salt, black pepper powder, turmeric and ground sumac to the bowl (knead the mixture thoroughly after adding each ingredient). Add 2 to 3 teaspoons of the soaked saffron to the bowl. Knead for 7 minutes. Cover the bowl with a lid or plastic wrap. Keep the beef mixture in refrigerator to marinate for 8 to 12 hours.
Note: For the best result use koobideh skewers to grill the kabobs (koobideh skewers are long and flat and wider than normal skewers). Place a fistful of beef mixture on a skewer and flatten. Grill the kabobs on the grill. Turn the kabobs often to avoid burning. Grill a few tomatoes and peppers to serve on the side. Kabobs can be served with Persian rice.