For the meat: Combine the sumac, cumin, coriander and paprika in a small bowl. In a larger bowl. combine ¾ of the spice mixture, the onion, garlic, salt and chicken and use your hands to thoroughly incorporate. Refrigerate the chicken mixture for 2 hours and up to 4. If using wooden skewers, soak them in water for an hour before cooking
While the meat is marinating, make the tzatziki and stir in the ground cumin.
Shape each kabob onto two skewers (you should get five or six kabobs from 1 pound of chicken) and place on a plate or pan coated with olive oil; turn the kabobs to coat both sides with a light layer of oil. Before grilling, sprinkle both sides with the remaining ¼ portion of the spice mixture. Grill over a medium flame for roughly 4 minutes per side (depending on the thickness of the kabob and the heat of the fire) until cooked through
Stir together the lemon juice and zest, sumac, olive oil, salt and pepper and toss with the greens, mint leaves, cilantro leaves, radish, cucumber and scallions and pile on next to the kabobs. Serve with the yogurt sauce on the side.