In a small bowl, mix together all the marinade ingredients.
Heat one side of a bass grill to medium. If you have a grill with charcoal, let it burn until the charcoal is covered with white ash and about medium hot.
While the grill is heating remove the giblets (if there are any) from the cavity of the chicken. Flip the chicken onto its breast. using poultry shears, cut down both sides of the backbone from tail to neck; discard backbone (or sometimes I save to make homemade stock!).
Open the bird onto your work surface, breast side up. Make sure that the legs returned inward. Using your fist or mallet, wallop the bird on the breast, hard enough to dislodge the bones and flatten out the breast. (Reserve a little to baste with while cooking.
Smear both slides of the chicken with the marinade. Lay over indirect heat on the grill. Cook, basting from time to time with remaining marinade until the juices run clear when a thigh is pierced deeply with a fork. If cooking on a gas grill, it will take about 45 minutes, if only a charcoal grill more like 1 hour 15 minutes - 1 hour 30 minutes, depending on size of chicken.
About 10 minutes before the chicken is ready, put the knob onions on the grill, turning frequently until softened and browned.