Prick bratwurst with a fork to prevent them from exploding as they simmer. In a large pot add bratwurst, combine beer, butter, and onions. Bring to a boil, reduce heat and simmer for 20 minutes. Remove from the pan with a slotted spoon.
Lightly oil the grill grate. Heat the grill to medium-high heat. Place bratwurst onto the grill and grill for 10-14 minutes, turning occasionally until browned on all sides.
Place bratwurst into the hoagie rolls and top with onions.