Mix 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 hot dogs to skillet. Cook 10 minutes; stirring occasionally.

Remove hot dogs from skillet and place directly on grill; cook 2 to 3 minutes, basting with sauce from skillet. Meanwhile, toast rolls on grill, open-side down, about 30 seconds.

To serve, place hot dogs on rolls. Top with onions and drizzle with additional Mustard and desired condiments, if desired.