Slice off top of garlic heads; wrap in foil and bake in 350°F (180°C) oven until softened, about 50 minutes. Squeeze garlic cloves into large sealable freezer bag; stir in onion, Worcestershire sauce, vinegar, sugar, honey, mustard, pepper and salt.

Pierce roast all over with fork; add to freezer bag; seal, squeezing out as much air as possible. Refrigerate for 24 hours, turning occasionally.

Transfer marinade into saucepan; add 1 cup water and boil for 3 minutes. Set aside. For 3-burner barbecue, place drip pan under grill in centre of barbecue; add 1/2 inch (1 cm) water to pan. Light remaining 2 burners and heat barbecue to low (225°F/110°C). Seal soaked hardwood chips in foil packet and poke with holes to let smoke escape; place under grill over a lit burner.

Place roast, fat-side up, on grill over drip pan. Maintaining constant temperature, close lid and cook, turning every 2 hours and basting with marinade, for 5 hours.

Remove roast to cutting board; let stand. Meanwhile, boil remaining marinade until thickened and onions are softened, about 20 minutes. Serve with thin slices of warm brisket.