For the ras al hanout: Add the cumin, cardamom, cinnamon, paprika, nutmeg, cayenne, cloves, allspice, coriander, pepper and turmeric to a jar with a lid. Cover and shake to combine. Measure out 1 tablespoon for the chicken and put in a large bowl. Store the remaining spice for up to 3 months in the jar and use as desired.
For the chicken: In the bowl with the ras al hanout, add the yogurt, mint, oil, salt, sugar, harissa, garlic, finely chopped onion, juice from half of the lemon and about 12 turns of pepper (about 1/2 teaspoon) and mix to combine. Fold in the chicken to evenly coat, then cover and let marinate in the refrigerator for at least 3 hours and up to overnight.
Soak 8 wooden skewers in water for at least 30 minutes.
Thread the chicken pieces on the skewers, adding the bell pepper and onion pieces in between the meat.
Heat a grill or grill pan over medium-high heat. Cook the skewers, turning them occasionally, until they're deeply browned on all sides and cooked through, about 7 minutes per side. Squeeze with the juice from the remaining lemon half before serving.