In a small bowl, combine the first six ingredients. Pour 1-3/4 cups into a large resealable plastic bag; add bratwurst. Seal bag and turn to coat; refrigerate for 4 hours or overnight. Cover and refrigerate remaining marinade.
Add onion to remaining marinade; toss to coat. Place on a double thickness of heavy-duty foil (about 18 in. square). Fold foil around onion mixture and seal tightly. Drain and discard marinade from bratwurst.
Grill bratwurst and onion, covered, over medium heat or broil 4 in. from the heat for 15-20 minutes or until meat is no longer pink and onion is tender, turning frequently. Open foil carefully to allow steam to escape. Serve brats in buns with onion mixture.