Season the chicken pieces with salt and pepper. In a medium, enameled cast-iron casserole, combine the chicken with the coarsely chopped onion and the chicken stock and bring to a boil. In a small bowl, mix the saffron threads with the ginger, coriander, cumin, paprika and turmeric. Stir the spice mixture into the broth. Cover and simmer over low heat, turning the chicken pieces once, until the breast pieces are just white throughout, about 25 minutes; transfer the breast pieces to a bowl and cover. Continue to simmer the drumsticks and thighs, covered, until done, about 15 minutes longer; transfer to the bowl with the breast pieces and keep covered.
Add the minced onion, the tomatoes, preserved lemon and artichoke hearts to the casserole and simmer over moderate heat until the broth is richly flavored, about 5 minutes. Season with salt and pepper and add the peas. Return the chicken to the casserole and simmer gently, turning a few times, until heated through. Serve the tagine in shallow bowls.