In a large mixing bowl combine the Greek yogurt, tahini, lemon juice, minced garlic, cumin (or za'atar), flaky sea salt and black pepper; stir until smooth.
Spoon out about one-third of the mixture into a small bowl and reserve it for serving as a sauce.
Pat the chicken thighs dry with kitchen paper, add them to the large bowl with the remaining marinade, and coat thoroughly; refrigerate for at least 30 minutes or overnight for best flavor.
Preheat the oven to 200°C (400°F). Lightly oil a baking sheet or spray with oil.
Arrange the marinated chicken on the prepared baking sheet so pieces do not touch; add the halved lemon, halved garlic bulb, and optional tomatoes to the tray.
Drizzle the chicken and vegetables with the tablespoon of olive oil and roast in the preheated oven for 25–30 minutes, until chicken is no longer pink and has a golden crust (internal temperature 79°C / 175°F).
Remove from the oven and serve the chicken with the pan juices; drizzle with the reserved tahini sauce and, if desired, squeeze roasted lemon and squeeze out roasted garlic cloves to serve.