Preheat BBQ to 375°F
Place beer, onions and bratwurst in an aluminum pan and place on BBQ. Make sure the sausages are completely covered in liquid, add more beer if needed.
Close lid and allow beer to come to a simmer
Allow to simmer for 10 minutes, stirring to make sure sausages remain covered in liquid.
Remove sausages from liquid and grill for 3 minutes, turn and grill additional three minutes
While sausages are grilling, stir the beer and onions, keeping it on the heat so that the liquid reduces to create caramelized onions.
Once beer has reduced, place the sausages back in the pan with the onions.
Turn the sausage in the pan to coat with the beer and onion mixture.
Toast the sausage buns on the grill.
Place 1 sausage on each toasted bun, top with onions and mustard.