In a large bowl, combine the cooked risotto with grated Parmigiano-Reggiano cheese. Season with salt and pepper to taste.
Take a small handful of the risotto mixture and flatten it in your palm.
Place a piece of mozzarella cheese in the centre, then shape the risotto around the cheese to form a ball (about the size of a golf ball).
Repeat with the remaining risotto and mozzarella, placing the formed balls on a baking sheet.
Set up a breading station with three shallow bowls: one with flour, one with beaten eggs, and one with breadcrumbs.
Roll each arancini ball in flour, then dip in beaten eggs, and finally coat with breadcrumbs, pressing gently to adhere.
In a deep pan, heat vegetable oil over medium heat until hot (about 180°C).
Fry the arancini in batches, turning occasionally, until golden brown and crispy, about 3-4 minutes.
Use a slotted spoon to transfer the fried arancini to a paper towel-lined plate to drain excess oil.
Arrange the arancini on a serving platter and garnish with fresh parsley if desired.
Serve warm with marinara sauce for dipping.