Heat the oven to 450°F and place a rimmed sheet pan inside to preheat.
Toss the chicken with olive oil, cumin, paprika, coriander, salt and black pepper and let it stand for 20 minutes.
Arrange the thighs on the hot pan and roast for 18 to 22 minutes, turning once, until browned at the edges and 165°F inside.
Whisk the tahini, lemon juice and garlic with 3 tablespoons water until smooth and spoonable.
Rest the chicken for 5 minutes, slice it thinly and toss the cucumber with tomato.
Cut the warm pitas in half, divide the chicken among all 8 pockets and add 1/2 cup cucumber-tomato mixture and 2 tablespoons tahini sauce across the 2 pita halves in each serving.