In a medium bowl pour in the grape jelly and mustard. Stir together. Spray a deep baking dish with cooking spray. Cut the kielbasa into 1/4-1/2 inch slices. Put kielbasa into baking dish, pour jelly mixture over the kielbasa. Bake at 375 degrees until the sauce has caramelized the kielbasa on the top. Make sure you stir the dish a couple times during cooking. Cooking time could be between 45 minutes and an hour.