In large zipper-closure food storage bag, marinate brats and onions in marinade; refrigerate several hrs. or overnight. Remove brats and onions from marinade. Preheat a grill to medium-high; grill brats over direct heat until cooked through and browned (5-6 min.), turning several times. Meanwhile, grill onions in grill basket until softened (5-6 min.), stirring several times. Cut a slit through the top of each bun, halfway through. Cut each brat into thirds; tuck a piece of brat inside each bun. Top with onions and a dollop of mustard.