Bake 16 to 18 min. or until done (160ºF). Meanwhile, melt 1 Tbsp. butter in same skillet on medium-low heat. Add onions; cook 10 min. or until tender, stirring occasionally. Add broth, dressing and mustard; mix well. Simmer 8 min. or until slightly thickened, stirring occasionally. Remove from heat. Add remaining butter; stir until melted.