1. In a bowl, whisk oil, 1/2 cup lemon juice, cumin, coriander, paprika and garlic; pour half into a separate bowl. Add chicken to one bowl; turn to coat. Toss peppers, zucchini and squash in other bowl. Cover bowls; chill for at least 2 hours.
2. Make sauce: In a bowl, whisk yogurt, tahini, 1 Tbsp. lemon juice and 1/2 tsp. salt. Cover and chill.
3. Preheat a gas grill to medium-high. Remove chicken and vegetables from marinade; pat dry and discard marinade. Thread chicken, vegetables and onions onto 12 metal skewers. Season kebabs with salt and pepper.
4. Lightly oil grill rack. Grill kebabs, turning, until chicken is cooked through, 4 to 6 minutes. Set aside 6 kebabs to cool. Serve remaining kebabs over brown rice, if desired, with sauce. When reserved kebabs have cooled, remove chicken and vegetables from skewers, wrap well and refrigerate.