Heat oil in a large pan, add the onion and cook for about 5 minutes until the onion starts to soften. Add in the mushrooms and garlic, stir and cook for a further minute.
Add in the rice and stir until the oil has coated the rice. Add in the wine and stir. Wait for the wine to almost fully absorb (stirring regularly) and then add the stock a ladle at a time. Ensuring each ladle has almost fully absorbed before adding the next. Ensure you stir regularly. This should take for 15-20 mins. Once you've used up all the stock, have a taste of the rice. It should be soft, but still have a bit of a bite to it. If the rice is still far too hard, you can add in a ladle or so of hot water. When the rice is cooked, add in the cream, Grana Padano cheese, lemon juice and salt & pepper. Stir and then turn off the heat. Leave to cool in the pan, then cover with clingfilm and refrigerate for 30 minutes (or up to 2-3 days).
When you're ready to make your arancini put the beaten eggs, breadcrumbs and flour on 3 large plates.
Take the cold risotto out of the fridge and take a heaped tbsp of the rice in your hand. Flatten it out and then place a folded up piece of the sliced Prosciutto di San Daniele in the centre. Wrap the risotto around the Prosciutto di San Daniele and roll into a ball. Place the ball onto a plate and repeat until you've used all of the rice.
Fill a large heavy-bottomed pan with the vegetable oil until it's about one-third full - the oil needs to be deep enough to cover the arancini. Heat the oil on high.
Dip each arancini ball in turn into the flour, egg and finally the breadcrumbs, shaking off any excess as you go.
Check the heat of the oil by dropping in a few of the panko breadcrumbs. If they immediately rise to the top and bubble vigorously, then the oil is ready. Turn it down to medium-high and carefully place in 4-6 arancini balls (depending on the size of your pan) in the oil. Cook for 2 minutes until the arancini are golden brown, then remove from the oil with a slotted spoon and place on a wire rack with some kitchen roll underneath to drain. Repeat until all of the arancini are cooked.
Serve with a green salad and extra Prosciutto di San Daniele and a sprinkling of grated Grana Padano.
Nutritional Information is per arancini, based on getting 16.