Combine half the lemon juice with 2 cloves grated garlic, about 1/3 cup EVOO, the sumac (or paprika mix), oregano, coriander and 1/2 tsp. cumin. Season the chicken or fish with salt and pepper, then let stand in marinade for 15 minutes.
Salt the cucumber in a colander; let rest for 10 minutes. Press out the excess liquid. In a food processor, combine the cucumber, yogurt, remaining lemon juice, 1 clove grated garlic, remaining 1/2 tsp. cumin and fresh herbs. Puree, then season the tzatziki.
Heat a skillet or griddle pan over medium-high heat. Shake excess marinade off the chicken or fish and place in the skillet; cook, flipping once halfway through, for 10 minutes or until cooked through. Slice the chicken on an angle or flake the fish.
Serve the chicken or fish with lettuce for wrapping. Top with some tzatziki, pepperoncini and tomatoes.