In a disposable roaster pan (I used an 11 ¾ x 9 ¼) whisk together beer, mustard, caraway seeds, and ½ teaspoon pepper.
Slice onions and lightly brush them with cooking oil, and then sprinkle them with pepper; place the pan on one side of the grill. Grill onions until slightly charred on both sides and then add them to the beer mixture.
Place brats on the grill and brown on both sides. Arrange the brats in the onion mixture. Close grill and cook brats until done, and onions are tender.
While brats are cooking place buns on grill cut side down to toast them; set aside.
Remove brats from the roasting to the pan to grill grates, and grill them until slightly charred on both sides. Continue cooking the onions in the sauce until the sauce starts to thicken.
Serve the brats on the buns with onions and sauce and your choice of condiments.