Rub Chicken With Oil; Sprinkle With Pepper And Salt. Place In A Greased 11-In. X 7-In. X 2-In. Baking Dish. Bake, Uncovered, At 425 Degrees F For 15-20 Minutes Or Until Juices Run Clear. Meanwhile, In A Small Bowl, Combine Cornstarch And Sour Cream Until Smooth; Set Aside. In A Small Saucepan, Combine The Broth, Grape Juice And Onions. Bring To A Boil; Cook For 4-5 Minutes Or Until Liquid Is Reduced To 1 Cup. Gradually Whisk In The Sour Cream Mixture. Bring To A Boil; Cook And Stir For 2 Minutes Or Until Thickened. Stir In Mustard Until Blended. Serve Over Chicken. Sprinkle With Chives.