Remove giblets and reserve for other uses; rinse chicken and pat dry.
Rub cut garlic over chicken skin, then put garlic in body cavity with onions and bay leaves.
Combine butter, pepper, thyme, oregano, marjoram, basil, sage, and sherry; spoon 1 tablespoon of the mixture over onions in chicken cavity.
Reserve remaining mixture for baste.
Close body cavity and secure with metal skewers; tie legs together.
Also secure neck skin across opening with a skewer and tuck wing tips under, akimbo-style.
Bank about 20 low-glowing coals on each side of fire grill and place a metal drip pan in center.
Place grill 4 to 6 inches above drip pan; grease grill lightly.
Brush bird with baste and place, breast-side-down, on grill directly over drip pan.
Cover barbecue, adjust dampers according to manufacturer's directions, and cook for 30 minutes.
Turn chicken breast-side-up and cook, basting occasionally, for about 30 minutes longer or until meat thermometer inserted in thigh (but not touching bone) registers 185°.
Scoop out onions from chicken; discard bay leaves and garlic before offering onions with the meat.