To cook bratwurst in beer, place 1/2 stick of butter, a medium onion and a dozen brats in a Dutch oven, and add enough beer to cover everything. After the beer reaches a boil, reduce the heat so that it simmers until the brats are thoroughly cooked. Transfer the brats to a hot grill until they get a light char on the outside. Place the brats back in the beer mixture until you serve them.
If you want to do all of the cooking outside, place the butter and onions in a disposable foil roasting pan with high sides. Let the butter melt and the onions soften for about three to five minutes. Add the beer, and put the pan on a hot grill until the beer simmers. Place the brats in the beer, and let them cook as you would on the stove top until they are cooked through.
Move the pan of beer to a warm area of the grill. Move the brats to the grilling surface, and cook them on the grill until they are charred. Transfer the grilled brats back into the pan of beer until you are ready to serve the meat. When handling the beer brats, use tongs instead of a fork so that you don't pierce the skin. This helps keep the juices inside the brats so that they stay moist.