You need:
1 l milk (full fat or semi skimmed)
4 tb spoons olive oil and more to fry them
1 leek chopped thinly
3 table spoons of flour
300 gr of serrano ham chopped very thinly (parma ham or equivalent is also good). You can do this in a food processor (for other kinds of croquetas substitute the ham by 300 gr of boiled or roast chicken, or 200 gr of raw prawns, or 4 boiled eggs)
2 slices of mild Cheddar cheese (or
salt
a pinch of nutmeg
To coat the croquetas (this is just indicative - add more if you need it):
1 glass of flour
1 beaten egg with 1/2 glass of milk
1 glass of fine breadcrumbs
Heat the oil in a deep pan over medium heat. Add the leek, reduce the heat and cook it for 10 minutes until it becomes translucent. Stir in the flour and continue stirring it for 5 minutes until the flour is golden. Separately (in the microwave if you wish) warm the milk and pour it into the flour mixture. Cook, stirring continuously, for about 2 minutes. Take the pan off the heat. Now, this is the trick for smooth velvety croquetas: with a hand held blender bled the mixture well for 3-4- minutes.
Add the ham, salt and the nutmeg. Take the pan back to the heat and keep stirring until you have a thick bechamel sauce (this should take another 4 minutes more or less). As you take the pan off the heat add the cheese. It should melt immediately. Stir it all once more, pour the mixture over a shallow dish and let it cool for at least one hour (or overnight)
Put three dishes in a row. Pour the flour on the first one. On the next one pour the beaten egg and the milk. In the last one add the breadcrumbs.
Pick a spoonful of the mixture (it should not be sticky - if it is, then let it cool a bit longer). With your hands mould the mixture like a kernel or a cylinder. Keep doing this with the rest of the mixture. You should get 45 to 50 croquetas.
Roll the kernels/cylinders in the flour, then in the egg and milk, and finally in the breadcrumbs. Put the croquetas on a tray (they should not touch each other) in the fridge so that they get firm before frying them. The croquetas are even better if you freeze them at this point - they last for 2-3- months in the freezer.
In a small deep frying pan or in a fryer heat olive oil over strong heat. The oil should cover the croquetas. Add 5-6 croquetas (you need to fry them in badges) Reduce the heat to medium and and fry for 2-3 minutes until they are golden. As you take them out put them over a kitchen paper lined tray to get rid of excess oil. Serve them immediately.
Kids like helping with making the kernels and with rolling the croquetas. Do not let them approach the frying pan as the oil is too hot and it will splatter if accidentally some of the mixture gets out of the coating.
I do not know any children who do not like croquetas.