Place a dutch oven or deep skillet over medium-high heat. Add butter and onions and allow the onions to soften just a bit. Add the bratwurst, bring to a boil, reduce heat, and simmer until brats are cooked through, about 10 minutes. Remove brats to a bowl or platter. Continue cooking onions in beer while grilling the brats (use as a topping if desired). Preheat gas or charcoal grill to medium-high. Remove brats from the beer mixture and grill them until golden brown. Return them to the beer mixture until ready to serve. Serve in a bun with sauerkraut, onions, mustard, and your other favorite toppings.