Submerge the onion in vinegar in a bowl for 10 minutes. Then drain well and pat dry with a paper towel.
Cook the bratwurst on the grill. Get nice grill marks on each sides and cook to an internal temperature of 155 degrees using a thermometer.
Optional: Scoop out a little of the inside of the roll if you desire it to be less doughy and cut the roll to fit the bratwurst if you desire less bread.
Place the cooked bratwurst on the roll, top with onion and mustard.