To prepare the marinade, first combine the Aleppo pepper with a tablespoon of warm water to form a paste. Set aside for about 5 minutes.
Meanwhile, mince the garlic and place into a large bowl. Next add the oregano, tomato paste, lemon juice, red wine vinegar, salt, pepper and whisk to combine. Next, add the olive oil, yogurt, Aleppo pepper mixture and whisk again. Set aside while you prepare the chicken.
Cut the chicken into approximately 1 1/2" -inch cubes. Add the chicken to the marinade and fold to evenly coat. Cover and let refrigerate for at least 4 hours, preferably overnight.
If using wooden skewers, soak them in cold water at least an hour before cooking.
Thread the chicken on to the skewers, allowing the excess marinade to drip off. Discard the leftover marinade.
Set up the grill for direct grilling and preheat it to high. Once hot, brush the grate clean. Using tongs, oil the grate liberally with a paper towel that has been doused in oil.
Allow the excess marinade to drain from the kebabs before placing them onto the grill. Grill until light golden brown and cooked through, about 2 to 4 minutes per side.
Remove from the grill. Sprinkle with sumac and/or more Aleppo pepper to taste. Serve hot.
These kebabs can be served with warm pita bread or Iranian flat bread (minus the cheese and basil), hummus and a nice green salad. Rice pilaf finished with fresh lemon and parsley also goes very well with these kebabs.