In a mixing bowl, whisk together all ingredients for yogurt sauce. Cover and place in refrigerator overnight.
In a resealable bag, add buttermilk, garlic, lemon juice, coriander, paprika, chili flakes, salt and pepper. Mix well. Add chicken and toss until coated. Refrigerate 8 hours or overnight.
Preheat oven to 425 degrees. Generously spray a sheet tray.
Remove chicken thighs from marinade and place on sheet tray. Roast in oven on top rack for 15 to 20 minutes, until top of chicken is charred. Remove and let sit for 5 minutes. Slice chicken in thin pieces.
Wrap flatbreads in foil and place in oven.
To assemble: Lay flatbreads on a work surface. Spoon yogurt tahini sauce on top. Divide chicken into fourths. Sprinkle with lettuce, tomato and cucumbers. Roll and serve.