In a large mixing bowl, combine the cooked Arborio rice, cubed mozzarella cheese, grated Parmesan cheese, chopped parsley, egg, salt, and black pepper. Mix well until everything is thoroughly combined.

Take a small amount of the rice mixture (about the size of a golf ball) and flatten it in the palm of your hand. Place a cube of mozzarella in the center and mold the rice around the cheese to form a ball. Ensure the cheese is completely enclosed. Repeat this process until all the mixture is used.

Set up a breading station: place the all-purpose flour in one shallow dish, the breadcrumbs in another, and keep a plate ready for the coated arancini.

Roll each rice ball in the flour, ensuring it is lightly coated. Shake off any excess flour, then dip the ball into the breadcrumbs, pressing gently to adhere. Place the coated arancini on the prepared plate. Continue until all the balls are coated.

In a deep frying pan or pot, heat oil over medium heat until it reaches 350°F (175°C). Carefully add a few arancini at a time, frying them in batches to avoid overcrowding.

Fry the arancini for about 3-4 minutes on each side, or until they are golden brown and crispy. Use a slotted spoon to transfer them to a paper towel-lined plate to drain excess oil.

Serve hot with marinara sauce or your favorite dipping sauce.