Cook the rice in plenty of salted, boiling water for 15-18 minutes until tender
Drain and tip into a bowl, stir in half the butter and Parmesan cheese
Spread the rice out on a work surface and leave to cool
Melt the remaining butter in a saucepan, add the beef and cook, stirring frequently, until browned
Sprinkle with the wine and cook until it has evaporated
Stir in the tomato purée, cover and cook over a low heat for 15 minutes
Season with salt and remove from the heat
Shape the cooled rice into croquettes as large as small oranges and hollow out the centres
Fill with a little meat sauce and a cube of mozzarella and seal with more rice
Beat the eggs with a pinch of salt in a shallow dish
Spread out the flour in another shallow dish
Dip the croquettes in the beaten eggs, then in the flour and shake off any excess
Heat the oil in a deep-fryer or saucepan to 180-190°C/350-375°F
Deep-fry the croquettes in the hot oil until golden brown all over
Drain on kitchen paper and serve