STEP 1: Melt the butter in a pan and stir in the flour to make a roux (thick paste).
STEP 2: Gradually stir in the milk until you have a smooth sauce. Simmer for 10-15 minutes. Add the cheese and stir until melted, then stir in the chorizo and season really well. Scoop into a tray or dish, cool, then chill completely in the fridge. (This can take 2-3 hours, or you could do it the day before.)
STEP 3: Beat the eggs in a shallow bowl and pour breadcrumbs onto a plate. With an ice-cream scooper or large spoon, scoop out the mix and roll each into a ball about 3cm across. Roll the balls in the egg then the crumbs. Repeat so you have a double layer of egg and breadcrumbs.
STEP 4: Fill a pan 1/3 full with oil and heat to 180C. Deep fry the croquettas in batches for 2-3 minutes until golden. Scoop out and drain on kitchen paper. Serve with alioli sauce.