Heat the butter in a pan over a medium-low heat, and fry the onion for 8 mins until softened but not coloured. Stir in the flour until it’s incorporated into the fat and onions. Reduce the heat to low and cook for 4 mins, stirring – don’t let it brown. Warm the milk in a medium pan over a low heat until steaming.

Remove the onion mixture from the heat and add the milk, a little at a time, stirring after each addition until smooth. When all the milk has been added, return the pan to medium-low heat and bring to the boil, stirring constantly. Reduce the heat to low and cook for 6 mins, stirring often to prevent it from catching. Set aside.

Cook the chorizo in a frying pan over a medium heat for 3 mins – you don’t need any fat, as fat will be released from the chorizo as it cooks. Stir the fried chorizo into the sauce along with the cheese, parsley and seasoning. Pour the sauce into a dish and leave to cool completely, then cover and chill for at least 3 hrs, or overnight.

Next, make the aïoli. Put the garlic and the egg yolks in a bowl and beat with an electric whisk while slowly adding the oil, a drop at a time. As the mixture starts to thicken, you can add the oil more quickly, in greater amounts. Squeeze in some lemon juice and season, tasting as you go to ensure the flavour is balanced (you can also add a drop of white wine vinegar as well as lemon juice, if you like). Cover and chill until ready to serve.

Using wet hands, roll the chilled chorizo mixture into walnut-sized balls. Put the egg in a shallow bowl and the breadcrumbs on a shallow plate. Roll the croquetas in the egg, then crumbs, making sure each is well coated. Put the croquetas on a baking tray, then cover loosely and chill for a few hours to firm up, so they don’t break up when you fry them.

Pour the groundnut oil into a pan until it is no more than a third full, and heat to 180C, or until a cube of bread dropped in turns golden within 30 seconds. Deep-fry the croquetas in batches for 2-3 mins per batch, keeping an eye on the temperature. If the oil is too hot, the croquetas will darken on the outside before the inside is hot – they should be golden. Use a slotted spoon to transfer the croquetas to a plate lined with kitchen paper, leave to drain for a few minutes, then serve with the aïoli.