Steam the potatoes in their skins (aka jackets) for about 30-40 minutes, or until they are fully cooked. (To test for doneness, stick a paring knife into them. If you can lift the blade out of the potatoes without lifting the potato, they're done.) Let the potatoes cool completely.

Peel the potatoes and cut them halves or quarters and pass them through a food mill or potato ricer.

Then mix in the rest of the ingredients listed above with a wooden spoon or spatula, making sure to salt them generously, until you have a stiff dough that forms a ball:

If the mixture feels a bit too soft or wet, add more grated cheese or a spoonful of flour, enough to stiffing things up.

Now set up yourself up with plates full of egg and breadcrumbs, plus your filling if using (today, I used mozzarella). Take a piece of the dough, about the size of ping pong ball, in your hands, and flattened it into an oval shape in the palm of your hand. If you are using a filling, press the filling into the center and close the dough around the filling and then roll it around with your hands to form a lozenge-shaped croquette. If not using the filling, just form the dough into croquettes.

Pass each croquette in the beaten egg, then in the breadcrumbs. When you are done, put them into the fridge for an hour or so, or until you are ready to cook.

Deep fry the croquettes in abundant oil, heated to 180C/350F, until they are nice and golden brown to your liking. This should take about 5 minutes.