Set up a grill for direct cooking over high heat. Melt the butter in a large skillet on the grill or side burner. Add the onions and cook until soft, about 10 minutes, stirring occasionally. Add the beer and cover; cook about 10 more minutes. Uncover the skillet and simmer until most of the beer has evaporated, about 5 minutes.
Reduce the heat to medium and oil the grates. Grill the brats on the grates 4 to 5 minutes per side. Lightly toast the rolls on the grill. Serve the hot brats on the rolls, smothered with the onions and topped with mustard.