Melt the butter and oil in a pan.
* Add the diced onion and sauté until it just starts to colour.
* Add nutmeg.
* Add the diced ham and sauté for 30 seconds more.
* Add the flour and stir continuously for around a minute.
* Add the milk little by little, always stirring until you incorporate the entire amount.
* Turn off the heat and let the dough cool to room temperature.
* Shape the dough into little sausages.
* Switch on your deep fryer, or pot with cooking oil.
* Take one of the croquetts, cover in flour, then egg, then breadcrumbs.
* Fry the ham croquettes in the hot oil for about five minutes until golden.