Set up grill for direct cooking over high heat. Oil grill grate when ready to start cooking.
Place a large skillet on a grill or side burner and add butter. When butter has melted, add onions. Cook until softened, about 10 minutes. Add beer and cover; cook an additional 10 minutes.
Remove cover from skillet and let simmer until most of the liquid has evaporated, about 5 minutes.
Turn the heat on the grill down to medium. Place brats on oiled grill grates and cook 4 to 5 minutes per side
Serve brats hot on toasted rolls smothered with onions and topped with mustard.
INDOOR: In a large skillet over medium heat melt butter add brats and brown 10 minutes, turning them occasionally as they brown. Remove the brats and set aside. Add the onions and cook for 10 minutes, stirring frequently until softened. Pour in the beer and scrape up any brown bits in the bottom of the pan. Return the brats to the pan and cook over medium heat for 15 minutes or until beer has all but evaporated.