Melt the butter in a large saucepan. Add the onion and cook over low heat for 3-4 minutes until softened but not browned. Heat the stock to simmering point in another saucepan.
Add the rice to the onion and cook, stirring for 1 minute to seal the rice. Add several ladles of the hot stick, stirring continuously so that the rice cooks evenly. Keep adding enough stock to just cover the rice, stirring frequently. Continue in this way for about 20 minutes, or until the rice is creamy on the outside but still al dente.
Remove from the heat and stir in the parmesan and eggs. Season with salt and pepper. Spread out on a large baking tray and cool completely.
Divide the rice into 18 portions. Take one portion in the palm of your hand and place a piece of basil and a cube of mozzarella in the centre. Fold rice over to encase the cheese and at the same time mould the croquette into an egg shape. Roll the croquette in breadcrumbs and place on a heavy baking tray while you make the rest.
Heat enough oil in the deep-fat fryer or deep frying pan to fully cover the croquettes. Heat the oil to 350 degrees F (180 degrees C), or until a piece of bread fried golden drown in 15 seconds when dropped in the oil.
Deep-fry the suppli in batches, without crowding, for about 4 minutes, or until evenly golden brown. Drain on paper towels and serve at once, as they are, or with a fresh tomato sauce.