How to cook
Chop the onion and fry it, until it's lightly browned in olive oil. Add the chopped spinach, salt and stew until it softens.
Pour the flour at once and stir for 1 minute, then pour the warm milk in two or three portions, while stirring constantly, until a thick bechamel sauce is obtained. You can add more milk, if necessary. Season with black pepper, crumble blue cheese and cook over low to medium heat, while stirring constantly, until the mixture begins to separate from the walls of the dish.
Remove it from the heat and cover it with cling film, so that it sticks tightly to the croquet mixture and so that a hard crust does not form. Allow it to cool and refrigerate it for 2-3 hours to overnight.
Prepare three plates for breading - one with the beaten eggs with a little salt, the other with flour and the third with breadcrumbs.
Form slightly elongated croquettes, by taking from the mixture as much as you can fit in a closed handful. If it sticks to you, wet your palms. Roll them first in the flour, then in the egg and finally in the breadcrumbs.
Fry the croquettes in heated oil over moderate to high heat. When they aquire a golden color, place them onto kitchen paper, so that it can absorb the excess fat.
Enjoy your meal!