Grab a handful of leftover risotto and shape it into a 2 1/2-inch diameter ball. Press a hole in the middle with your thumb and place a few cubes of mozzarella cheese inside. Reshape the rice ball, until the mozzarella is fully enclosed in the rice. In a deep saucepan, heat the extra-virgin olive oil to 350 degrees F. Roll each rice ball into the beaten egg, and then coat evenly with bread crumbs. Gently place the rice balls in the hot oil without overcrowding. Make sure the olive oil is hot before the rice balls are added; otherwise the rice balls will absorb the oil and become heavy. Fry the rice balls for a few minutes on each side, or until golden. Place the rice balls on the paper towels to absorb any excess oil. Transfer to a plate, and serve them hot or at room temperature.
*If leftover risotto is not on hand, add a generous amount of grated Parmigiano cheese and a small amount of tomato sauce to boiled, salted arborio rice.