The filling for Spanish croquetas is essentially a very thick béchamel sauce, with whatever you want added in, formed into rough cylinders, floured, egged, breadcrumbed and fried. Cured ham is the most common ingredient, but chicken and salt cod are other popular options. For veggies, mushrooms are another goodie, as you discovered. The best I have ever had, in Asturias on the northern coast in 'Green Spain' (a heavenly part of the world), were made with Cabrales blue cheese. I can still taste them - 25 years later.

The second best, in my meat-eating days, were made with butifarra negra (Catalan equivalent of morcilla, similar to British/Irish black pudding), in Girona (also worth a visit from Girona, just half an hour now on the AVE high-speed train).