For the croquetas, melt the butter in a large saucepan, then whisk in 200g/7oz of the flour and cook for 2-3 minutes. Whisk in the milk slowly until you have a very thick sauce. Season with salt and pepper. Divide into 2 bowls and in 1 bowl stir in the fresh breadcrumbs, ham, manchego and half the parsley, and in the other the cooked salt cod and remaining parsley. Place in the fridge to chill for 3-4 hours.
When you're almost ready to cook the croquetas, fill a deep-fat fryer or a large, deep, heavy-bottomed pan two-thirds full with rapeseed oil. Heat the oil to 170C (CAUTION: Hot oil can be dangerous. Do not leave unattended.)
Place the remaining flour, beaten eggs and dried breadcrumbs in 3 separate bowls. Shape the chilled mixture into 7cm/2¾in cylinders. Coat each one in the flour, then egg and finish with a layer of breadcrumbs.
Deep-fry the croquetas in batches until golden-brown. Remove and drain on kitchen paper.
For the aioli, blend all the ingredients, except the olive oil, in a food processor. Pour the oil into the blender in a steady stream, until it forms a thick sauce. The mixture, once blended, should be vibrant and yellow in colour. To vary the flavour, add a little mustard or some saffron. If you'd like your mayonnaise runnier, add a couple of tablespoons of hot water.
Serve the croquetas on a board with a bowl of the aioli for dipping.