How to Make It
Prepare grill for indirect grilling, heating one side to high and leaving the other side unheated.
Meanwhile, place the sausages, beer, and onion in a 4-quart ovenproof saucepan. (The beer should just cover the sausages; add more water if necessary.) Once grill is ready, place the saucepan in the middle of the grill, cover, and bring the beer to a gentle simmer. (Do not let the liquid boil or the sausages will split.) Adjust the saucepan as necessary, moving it to the cool side of the grill if needed to maintain a gentle simmer. Simmer for 20 minutes or until sausages are plump and opaque.
Move saucepan to the cool side of the grill. Using tongs, carefully transfer the sausages to the hot side of the grill, making sure not to puncture the casings. Grill 5 minutes or until sausages are browned and crisped all over, turning occasionally. Spread the ketchup and mustard evenly over buns; nestle 1 sausage in each bun. Serve with onions, if desired.