Combine grated zest and juice of 1 orange, onion, vinegar, garlic, rosemary, salt, pepper and ground cumin in large sealable freezer bag. Remove 1/4 cup (50 mL) of the marinade and set aside. Pierce roast all over with fork and add to bag; seal and refrigerate for 30 minutes or up to 8 hours.

Prepare barbecue: Place drip pan containing 1/2 inch (1 cm) water under grill. Preheat grill to medium-high 400°F (200°C). Remove roast from marinade; discard used marinade. Pat roast dry with paper towel and season all over. 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 on grill over drip pan that is moved to one side; turn heat off under just the roast.

Insert meat thermometer into centre of roast, avoiding spit rod (if using). Cook at constant heat, in closed barbecue, until thermometer reads 145°F(63°C) for medium-rare, about 1-1/4 to 2 hours. MEANWHILE, combine the reserved marinade with the mayonnaise to make the sandwich topping.

Remove roast to cutting board; tent with foil and let stand for 10 to 15 minutes. Thinly slice roast beef and prepare sandwiches or serve simply as sliced beef with some of the sandwich topping.