Using a big tin pan (use something that is safe for the grill), combine the onions and butter and place the bratwursts in the pan. Pour the beer over the brats and make sure the brats are submerged.
Using your outdoor grill, place the pan on the grill and bring to a boil. Let cook for 20-25 minutes and then remove from the pan and grill each bratwurst for at least 3-5 minutes on each side.
Place into a bun and top with mustard, sauerkraut, the red cabbage slaw and some grilled onions if desired. Enjoy!