|| Combine chicken, zest, juice, salt, pepper, garlic and olive oil in a non-metallic container. Let stand 15 minutes.
In a small bowl, combine yogurt, cucumber, mint, kosher salt and black pepper. Set aside.
||Preheat grill to high heat. Grill chicken 5 minutes on each side until grill-marked and cooked through (180°F). Grill red onion briefly on both sides.
||Slice chicken into 1/2 inch strips. Fill pocket bread halves with chicken, onion rings, tomato slices and spinach leaves. Drizzle with 1-2 tablespoons yogurt-cucumber sauce. Serve remaining sauce alongside for dipping.