Heat the olive oil in a pan over medium heat and sauté the onions until transparent. Stir in the flour and cook it briefly for 4−5 minutes. Whisk in the milk. Cook, stirring constantly until the sauce thickens, about 5−8 minutes. Add the Spanish paprika, cayenne, and smoked paprika. Season with salt and pepper. Stir in the spinach, Manchego, and feta and spread the mixture into a dish to cool. Refrigerate until solid.
Place the beaten eggs in one dish and the breadcrumbs in another. With moistened hands, form the chilled mixture into balls or cylinders. Dip each croqueta first in breadcrumbs, then in beaten egg, then in breadcrumbs again, taking care that they are well covered. Allow to cool in the refrigerator for 30 minutes.
Heat a large saucepot filled about halfway with oil to 350 degrees. Fry the croquetas a few at a time, until golden, about 2 minutes. Remove and drain on paper towels.
Serve with guava jam or another favorite jam or marmalade. Fig is also delicious.