Combine first 5 ingredients in a medium bowl, stirring with a whisk. Pour into a large zip-top plastic bag; add chicken. Seal bag, and marinate in refrigerator 8 hours, turning bag occasionally.
. Prepare grill.
. Remove chicken from bag, discarding marinade. Sprinkle chicken with salt, and coat chicken and onion with cooking spray. Place on grill rack coated with cooking spray; cook 6 minutes on each side or until chicken is done. Remove chicken and onion from grill; cut onion slices in half, and separate into strips.
. Combine lemon juice, oil, and cilantro in a medium bowl. Add onion strips; toss well. Spoon onion mixture onto a serving platter. Top with chicken.