In a small bowl mix the mustard ingredients. Cover and let stand at room temperature until ready to serve.
Prepare the grill for direct cooking over high heat (230°C to 290°C). If you’re using a charcoal grill, keep a small part of the charcoal grate clear of briquettes as a safety zone of indirect heat.
Brush the cooking grates clean. Put the hard cider, onions and bratwurst into a grill-proof roasting pan. Place the pan over direct high heat, close the lid and bring the liquid to a simmer. Continue simmering until the brats are evenly coloured and have lost their raw look, about 20 minutes, turning them occasionally. If the liquid starts to boil, move the pan over indirect heat to prevent the bratwurst from splitting open.
Lower the temperature of the grill to medium heat (180°C to 230°C). Transfer the brats to another grill-proof roasting pan. Strain the onions in a colander over the pan with the brats (the liquid will keep the brats warm while you cook the onions). Return the onions to the original pan and stir in the brown sugar. Cook the onions over direct medium heat, with the lid closed, until they are golden brown, about 15 minutes, stirring occasionally. Move the onions over indirect heat to keep them warm.
Brush the cooking grates clean. Remove the brats from the liquid and grill them over direct medium heat, with the lid closed, until browned, 6 to 8 minutes, turning once or twice. During the last minute of grilling time, toast the rolls over direct heat.
Place the brats in the rolls. Spread each with the mustard and top with the glazed onions and a few apple slices. Serve hot.