Drain off all but 1 tablespoon of fat from the pan, then add the onion and garlic, stirring and scraping to pull up any brown bits from the bottom of the pan. Cook, stirring frequently, until soft (about 3 minutes). Stir in the tomato paste, and cook for 1 minute. Stir in the chickpeas, harissa and chicken stock, and bring the mixture to a simmer. Nestle chicken, skin side up, in the chickpea mixture. Transfer skillet to oven, and roast until chicken is cooked through (20–25 minutes).