Preheat oven to 375 degrees. Arrange chicken pieces in a roasting pan and put one teaspoon grainy mustard on each one. Spread evenly over top. Sprinkle garlic and oregano over chicken and spoon diced tomatoes over the top. Pour tomato juice in bottom of pan. Cover with heavy foil and bake 35 minutes. Remove foil and bake 15 minutes more. Serves 4 – 8.