Heat the oil and butter in a medium, heavy-bottomed saucepan over a medium heat and, when the butter melts stir in the flour and cook gently, stirring regularly, until it loses its raw flavour – this should take about 4 minutes.
Gradually stir in the milk, beating it in well, until you have a smooth paste. Cook for another 5 - 7 minutes until it has the consistency of smooth mashed potato. Divide the béchamel equally into 3 bowls.
Into 1 bowl stir the butternut squash and fontina cheese.
Into the second bowl stir the pancetta and mozzarella.
Into the third bowl stir the chorizo and manchego.
Allow the croquette mixes to cool, then cover, pressing the clingfilm on to the surface of the sauce to prevent a skin from forming, and refrigerate for at least 2 hours.
Put the beaten eggs into a bowl, and the breadcrumbs and cheese into another. With floured hands, roll spoonfuls of the mixture into shapes (we rolled ours into cylinders, balls and squares) and dip these into the egg, then roll in the breadcrumbs until well coated.
Heat the oil in a large pan to 180C, or until it begins to shimmer. Prepare a plate lined with kitchen roll. Fry the croquettes in batches for a couple of minutes until golden all over, then lift out with a slotted spoon, sprinkle with salt and serve at once.