Chop the onion very finely
Fry the onion in oil and 2 tbsp of butter.
When it becomes transparent, pour in the rice
Add a pinch of salt and toast it for a couple of minutes, then add in a little vegetable broth and mix it in.
Add the tomato puree and hot milk.
Cook over medium heat for about 15 minutes, stirring constantly and adding more broth as often as needed. When the rice is almost cooked, add the chopped basil.
Season with salt and stir in the rest of the butter, pecorino, and Parmigiano.
Mix the ingredients well.
Spread the risotto on a cold surface.
Cut the mozzarella into cubes. With wet hands, take a spoonful of rice and form a meatball shape, in the center of which you will form a small cavity.
Place a cube of mozzarella inside and close by compacting the ball well so as to give it a slightly elongated shape.
Beat the eggs in a bowl with a pinch of salt and roll the supplì in it making sure to get all of the sides covered.
Pass them in the breadcrumbs, and make sure it adheres well over the entire surface
Fry them at 170-175°C (340°F) for about 3 minutes.
The supplì must become a light walnut color.
Blot with absorbent paper. The supplì need to rest for a moment: they must be served hot, but not too hot.