Preheat an outdoor grill for medium-low heat and lightly oil the grate.

Cook brats, covered, on preheated grill, turning often, until browned and an instant-read thermometer inserted into centers reads 160 degrees F (70 degrees C). Transfer brats to foil baking pan. Increase grill heat to medium.

Place foil pan on grill; add beer, onions, and butter. Cover grill; simmer brats. Serve brats and onions on buns.