Gently fry the chopped onion in a frying pan until it turns golden brown, then add the chicken, chopped into very small pieces. Add salt and pepper and cook for 5 minutes over a medium heat and put by.
To make the Béchamel sauce, first make a roux with the flour and butter by melting the butter and gradually stirring in the flour to make a smooth paste without any lumps. Now add the warm milk, stirring all the while. Leave to cool at room temperature, covered with clingfilm so it doesn’t dry out. When cold, add to the chicken.
You can make the croquettes straight away, but it’s better to let the mixture rest in the fridge at least overnight; that way, it will thicken and you’ll find the croquettes much easier to make. Simply shape them to the right size and then coat them in egg and breadcrumbs. Fry in plenty of very hot extra-virgin olive oil, making sure each croquette is completely covered by the oil. Don’t try to fry too many at once, or the temperature will drop and they won’t cook as well.