Brush both sides of onion slices with vegetable oil and season with salt and pepper.
In a disposable 13 x 9-inch aluminum pan, mix together beer, mustard, sugar, caraway, and 1 teaspoon pepper.
Place the aluminum pan on one side of the grill, and arrange onions on the other side and cook until the onions are lightly charred. Move onions into the aluminum pan and arrange sausage on the grill.
Cook sausage until browned on both sides, about 6-10 minutes. Move sausage to pan. Cover the grill, and continue to cook until the sausages have finished cooking, approximately 15 minutes.
Remove sausages from beer mixture and grill until they are lightly charred, about 4 minutes. Remove charred sausage to plate and tent with aluminum.
Continue to cook onions with grill covered, in mustard mixture, until they are soft and tender and beer mixture has thickened just a bit, about 5-7 minutes.
Optionally: Split open the rolls and lightly brush the entire inside with oil.Place on the grill and cook until slightly crisp.
Serve. Place charred Brats on rolls, top with your favorite toppings, and spoon onion-mustard mixture over them. Enjoy