MIX beer, Mustard and honey in medium bowl; set aside.
HEAT large cast iron skillet on grill over medium heat 3 minutes. Melt butter in skillet. Add onions; cook 2 to 3 minutes or until softened and lightly browned. Add sausages to skillet and stir in beer mixture. Cook 10 minutes; stirring occasionally.
REMOVE sausages from skillet and place directly on grill; cook 2 to 3 minutes, basting with some of the Mustard Sauce from skillet. Meanwhile, toast rolls on grill, open-side down, about 30 seconds.
TO SERVE, place sausages on rolls. Top with onions and drizzle with remaining Mustard Sauce from skillet to serve.