Preheat oven to 450 degrees. In 13 x 9 metal baking pan, toss potatoes and onions with olive oil. Salt and pepper to taste. Bake 25 minutes on middle oven rack.
Meanwhile, place chicken breast halves in small roasting pan. After vegetables have baked 25 minutes remove pan from oven and carefully turn vegetables. Return pan to oven, placing pan on lower oven rack. Place chicken on upper rack.
After chicken has baked for 10 minutes, remove from oven, brush with mustard. Bake chicken & vegetables 12-15 minutes longer until juices run clear when thickest part of chicken is pierced with a knife and vegetables are golden & tender.
Makes 4 main dish servings.