Finely chop the cooked ham in a food processor or mince it using the coarse blade of a mincer, and put the onion through the food processor or mincer also.
Then, in a large mixing bowl, combine the ham and onion. Soak the bread in the milk, then squeeze it out and stir this in, together with the garlic, parsley and mustard. Season with pepper, but be sparing with the salt, as the ham may be salty. Add the beaten egg and stir thoroughly. Now mould the mixture into 8 large - or 12 smallish - croquettes or rolls, pressing firmly to bind them together and chill them for 30 minutes. Then, sprinkle some flour on to a board and roll each croquette in it so that they're all lightly dusted with flour.
Shallow-fry them in olive oil for 5-6 minutes, turning them halfway, to a light brown colour.
Fried eggs would make a nice accompaniment.