In a large resealable plastic bag, combine the baharat, sumac, cumin, coriander, preserved lemon rinds, garlic, chicken stock, olive oil, salt, and pepper. Seal the bag and gently massage everything together. Open the bag and toss in the chicken thighs and onions, reseal, and massage again to fully coat the chicken. Toss the bag in the fridge for at least 2 hours but preferably overnight to marinate, turning the bag once or twice.