1. Place chicken in a medium bowl and add 1 1/2 cups of yogurt, 2 chopped garlic cloves, rosemary and lemon juice; let mixture marinate at least 15 minutes or up to overnight.
2. Coat a broiler pan or a stove top grill pan with cooking spray. Place grill pan over medium-high heat and cook chicken and red peppers, turning once, until chicken is cooked through, about 3 to 4 minutes per side. Or preheat broiler and broil chicken and red peppers, turning once, until chicken is cooked through, about 3 to 4 minutes per side.
3. Meanwhile, to make relish, combine cucumber, onion, remaining 1/2 cup of yogurt, remaining chopped garlic clove, parsley, oil, salt and black pepper in a medium bowl; serve relish over chicken. Yields about 1 1/2 cups of chicken-pepper mixture and 1/2 cup of relish per serving.