In a large bowl, mix together the mashed potatoes, mozzarella, flour, parmesan, parsley, pepper, and egg. Place the breadcrumbs on a large plate.
Form the potato mixture into 12 golf-ball-size balls (about 2 ½ tablespoons each). The balls will be a little sticky, so wet your hands a little as you go. Roll the balls in the breadcrumbs, pressing to make sure they adhere. Place the balls on a clean plate as you go.
In a deep skillet or Dutch oven, heat 2 ½ inches oil to 375°F, or until it bubbles when you add a small pinch of breadcrumbs.
Add the potato balls to the oil and fry until golden brown on all sides, 3 to 5 minutes, turning occasionally. Transfer to a paper towel-lined sheet pan or plate and immediately sprinkle with salt. Serve with more parmesan and parsley.
