Brown the chopped onion with the butter in a non-stick pan.
Add the rice and let it toast, then add the hot broth little by little and cook.
In the meantime, prepare a pistachio pesto: set aside 25 g of pistachios which you will crush into grains and blend the rest (50 g) with a drizzle of oil until you obtain a paste.
Finally, season with the pistachio pesto, the chopped nuts and the parmesan, mix well, add salt and leave to cool.
Cut scamorza and mortadella into cubes.
Take 1-2 tablespoons of rice at a time, place a few cubes of scamorza and mortadella in the center and close, creating a sphere.
Beat the eggs with milk and a pinch of salt and dip the arancini, one at a time, first in the eggs and then in the breadcrumbs.
Finally, fry them, a few at a time, in plenty of hot seed oil, turning them to brown them evenly.
Once golden brown, drain them with a slotted spoon on kitchen paper.
Your pistachio arancini are ready, serve them hot or at least warm.