HEAT oven to 400ºF.
ADD wieners to broth in medium skillet; bring to boil. Cook 5 min. or until wieners are heated through. Remove wieners; set aside.
STIR vegetables into remaining broth; cook on medium-high heat until broth evaporates. Add butter and sugar; cook 5 min. or until onions are browned, stirring occasionally.
MEANWHILE, spread cut sides of buns with mayo; place, cut-sides up, on baking sheet. Bake 3 min. or until lightly browned. Fill with wieners and onion mixture.