We used to have these any time there was leftover rice or risotto. Sometimes as small balls with cheese in the center or as a small patty. Great to start a meal off or just as an appetizer with some wine.
Heat the olive oil in a heavy-based saucepan over medium heat and add the onion and saffron. Saute until the onion is translucent.
Add the rice and saute for a minute or two, then pour in 2 cups of the stock. Bring to the boil, then reduce the heat to low. Cover and cook gently for 20 minutes or until the rice is tender, stirring from time to time and adding more stock if needed to prevent the pan from drying out.
Stir in the parmesan and butter, then season to taste with salt & pepper & stir until the butter has melted and the mixture is well combined. Spread the risotto on a baking tray and refrigerate until cold - preferably overnight. (Left over rice or risotto is always great to use).
Take a large spoonful of the cold rice and use the palms of your hands to form a ball the size of a golf ball (about 1-inch in diameter or so). Push a hole into the center of the ball and fill with a small chunk of the mozzarella, then re-roll the ball, ensuring the cheese is completely enclosed. Place the flour and eggs in separate shallow bowls. Roll the ball first in flour, then beaten egg and finally panko crumbs. Repeat with the remaining rice, mozzarella, beaten egg and panko crumbs.
Heat vegetable oil for deep-frying in a heavy-based saucepan to 350 degrees F. Fry the arancini in batches for 3-4 minutes until they are golden and crisp on the outside and the mozzarella is melted and stretchy on the inside.
Drain on paper towel - sprinkle with a little salt & serve immediately
NOTE: Leftover rice or risotto is always GREAT to use.