Rinse and trim chicken breasts and place in a 1 quart freezer ziplock bag or other covered marinade bowl. Add sliced red onions.
Add remaining ingredients and toss to coat chicken and onion rings. Refrigerate and let marinade at least 2 hours or overnight.
I like to put the chicken in the marinade without the onions and freeze for use during a weekday. If doing this, take out of the freezer the night before using and refrigerate. Add onions and toss to coat about 2-4 hours before grilling time.
Heat barbecue grill to medium high. Remove chicken and onions to the grill, reserving marinade to a small pan. You may want to use a vegetable tray or foil pouch with holes for the onions on the grill.
Heat reserved marinade and bring to a boil. Baste chicken with marinade and turn occasionally. Continue to cook chicken until cooked through and juices run clear, about 11 minutes.
Transfer chicken to serving plate with onions and pour over or serve with any remaining marinade.