In a large braised or sauté pan, sauté the sliced onion in butter until it is soft and translucent, making sure it does not brown. Season with salt. Add the rice and sauté that until it has turned a chalky white, then add a splash of wine and let it evaporate.

Bring the broth to a simmer and add the tomato paste until it is fully dissolved. Now proceed to add the broth, ladle by ladle, as if you were making a risotto, until the rice is tender—more than al dente. Taste and adjust for seasoning. Take the rice off the heat, add the grated cheese and mix well, then add the eggs and mix again. Immediately transfer the rice to a baking pan, spreading it out into a thin layer with a spatula. Let the rice cool entirely, about 10-15 minutes. (If you want to leave the rice longer than that, cover it with plastic wrap so it doesn't develop a 'skin'.)

When the rice has cooled, take up a handful in the palm of you hand, flatten it out a bit, and in the middle place a cube of mozzarella (or whatever other stuffing you are using—see Notes below). Close the rice around the stuffing, press it together firmly with your hand and, now using both hands, form the rice into a slightly oval croquette.

Roll the croquette in breadcrumbs, making sure they are well covered all over. (At this point, you can leave your croquettes until you are ready to eat them.)

Deep fry the croquettes in abundant oil until they are nice and golden brown all over, about 5 minutes or so. The oil should not be too hot—say, about 160C/320F—so it won't brown too quickly on the outside before the supplì have had the chance to cook on the inside.

Serve your supplì while they're still nice and hot.