In a casserole set over moderately high heat, warm 1 tablespoon of the oil until hot. Add the chicken and cook, stirring, until no longer pink. With a slotted spoon transfer to a plate. Add the remaining oil to the casserole, the onions, pepper, carrots, celery, garlic, salt, pepper, cumin, coriander, and bay leaves, and cook the mixture over moderate heat, covered, stirring occasionally, for 5 minutes. Add the tomatoes and cauliflower and cook, stirring, 3 minutes more. Add the water, bouillon cube, and lemon slices, bring to a boil, and simmer, covered, 5 minutes, or until vegetables are just tender. Return the chicken to the casserole, add the couscous, and cook over moderate heat, stirring, for 2 minutes. Add the Harissa paste and salt to taste, stir to combine and let stand, covered, off the heat 5 minutes. Remove and discard bay leaves before serving.