Pierce roast all over with fork. Place in large sealable freezer bag with dressing; refrigerate 12 to 24 hours. Place drip pan, containing 1/2″ (1 cm) water, under grill. Preheat barbecue to 400ºF (200ºC), using medium-high heat. Discard marinade. To Cook with Rotisserie: Insert spit rod lengthwise through centre of roast; secure with holding forks and place roast over drip pan. To Cook Without Rotisserie: Place roast directly on grill positioned over a drip pan that is moved to one side; turn heat off under just the roast.

Insert meat thermometer into middle of roast, avoiding spit rod (if using). Cook at constant heat, in closed barbecue, until thermometer reads 140°F (60°C) for medium-rare (about 30 min per lb/500g).

Remove roast to cutting board; cover with foil and let stand 10 to 15 minutes. Carve into thin slices to serve.