In a small bowl mix the mustard ingredients. Cover and let stand at room temperature until ready to serve.
Prepare the grill for direct and indirect cooking over high heat (450° to 550°F).
Brush the cooking grates clean. Put the hard cider, onions, and bratwurst into a large disposable foil pan. Place the pan over direct high heat and bring the liquid to a simmer. Keep the grill lid closed as much as possible. Continue simmering until the brats are evenly colored 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. Transfer the brats to another large disposable foil 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 as much as possible, until they are golden brown, about 15 minutes, stirring occasionally. Move the onions over in direct heat to keep them warm.
Remove the brats from the liquid and grill them over direct medium heat until browned, 6 to 8 minutes, turning once or twice. During the last minute, place the buns on the grill to toast.
Place the brats in the buns. Spread each with the mustard, and top with the glazed onions and a few apple slices. Serve hot.