Preheat your barbecue to medium-high. When hot, turn one side of the barbecue off; leave the other side set to medium-high. Split the buns and spread with mayonnaise and horseradish. Stuff in the cheese, roast beef, and onions. Wrap each bunwich in foil. Place the bunwiches on the unlit side of the barbecue, close the lid and cook for 5 minutes or until the sandwiches are nicely heated through.
Note: The buns used for this recipe were Thrifty hotdog buns which are baked in-store. The roast beef and havarti cheese are available in the deli service case.