In Spain, croquettes of all sorts--bacalao, jamón serrano, and even chicken--are extremely popular as tapas, but one of the most unusual and delicious is the mejillón tigre (literally, "tiger mussel"), which is basically a mussel croquette fried with one half of the shell. They seem to be especially popular in Madrid, where every neighborhood has a bar that makes the mejillón tigre one of its specialties. In barrio de Prosperidad, where we stay, it's Luman (where we took the picture above). If you're visiting the Prado, you may want to cross the street and try the mejillón tigre at Espacio Puro. They're fantasitc, and the place has a neat retro atmosphere.
Wherever you are, ask around and someone will probably point you in the direction of the best mejillón tigre.