In a large bowl, combine the yogurt, salt, ras al hanout, sugar, harissa, oil, onion, garlic, mint, juice from half of the lemon and about 12 turns of pepper (about 1/2 teaspoon). Fold in the chicken to evenly coat the cubes, then cover and let marinate in the refrigerator for 3 hours or overnight. Soak 8 wooden skewers, adding bell pepper and onion (if using) in between the pieces.