Croquettes are universally liked in Spain. Basically, you need a thick béchamel-flavoured sauce and filling of fish (shrimps, tuna or crab) or chicken, sweetcorn or mixed meat or whatever rolled in flour, dipped in eggwash and breadcrumbs and then deep-fried. You can make them any size or shape you like, from round to square, from tubes to ovals to little cylinders. They make wonderful tapas.
Be sure to deep-fat fry them and, as long as you have kept the mixture in the refrig