Combine all ingredients, except for the quartered peppers, onions, and tomatoes.
Heat a cast iron griddle or fire up the grill until very hot
Shape the kafta mixture into a long hot dog shape around the metal skewer, or bamboo skewer. Flatten it just a bit so that it is not too thick. This ensures even cooking on both sides.
Place the skewers of beef onto the hot grill, along with the quartered vegetables if using them.
Rotate the skewers after about 3-4 minutes, checking for grill marks. Remove after cooking on each side for 4 minutes, or until cooked through. Be sure not to overcook, as it will dry out.
Arrange the onion slices on a platter, sprinkle with sumac and chopped parsley. Arrange the cooked kafta skewers and charred vegetables over the onions and serve with pita.