Stir together apple, cheese, bread crumbs, mustard, 1 tbsp (15 mL) chives, and vinegar.
Preheat grill to medium-high; grease grate well. Butterfly sausages by slicing lengthwise without cutting all the way through; open flat. Grill sausages, cut sides down, for 5 to 7 minutes or until well marked.
Flip sausages; top evenly with apple mixture. Close lid and cook for 5 to 7 minutes or until cheese has melted and internal temperature has reached 165°F (74°C). Let stand for 5 minutes. Garnish with remaining chives.