First, make the mozzarella balls: wrap a small piece of mozzarella in 2 tablespoons of leftover stuffing or mashed potatoes, sealing completely.
Bread the mozzarella balls: coat lightly in beaten egg, then coat in panko. Set aside on plate until all are breaded.
Add the oil to a deep, heavy bottomed pan and bring oil to 350°. If you don't have a thermometer, flick a very small amount of water into the oil to see if it sputters. It's ready if it does!
Fry the mozzarella balls for about a minute on each side until golden brown. Transfer to a plate lined with a paper towel, and enjoy immediately!