Bring stock to the boil in a saucepan, add saffron if using and then the rice. Bring back to the boil and then turn down and simmer gently until the rice is cooked and all the stock is absorbed. Stir in parmesan, check for seasoning and add salt if needed. Spread out on a plate and put in the fridge to chill completely. Alternatively, just use leftover risotto.
When the rice is chilled, you can make arancini. Pinch up a small apricot sized piece of rice and roll into a ball. Poke your finger into the ball and using a finger and thumb, make the ball into a little pinch pot with sides of an even thickness. Don’t go too thin or your pot will crack when you fill it.
Spoon in about 1/3 of a teaspoon of pesto and a thumbnail sized piece of mozzarella to fill the cavity. Pinch off another small piece of rice, flatten it and use this to patch the top of the pot and make the ball round again. Roll very gently and pop on a plate while you finish the rest of the mixture in the same way.
Get three bowls ready for coating the arancini. In the first put flour, in the second two beaten eggs and in the third, breadcrumbs.
Dip each rice ball first into flour to coat completely, then quickly, but thoroughly coat with egg. Finally, drop into the breadcrumbs and use a teaspoon to sprinkle crumbs over it and jiggle the bowl a bit, until you can pick it up to check the crumb coating is even. Set aside and repeat with the rest of the balls. They can now be chilled for up to 24 hours before you fry them. Don’t leave them any longer though because they contain cooked rice.
To fry, simply heat your fat in a deep-sided saucepan until a small cube of bread browns in a minute. Deep fry the arancini in batches, turning to make sure that they are evenly browned, if the fat doesn’t completely cover them. They are done when the crumbs are evenly golden brown.
Drain on kitchen paper and keep warm while you cook the rest.
