To make the spinach croquettes, start by chopping the onion and washing the spinach.
Put the butter and oil in a frying pan. Then add the onion. Let it cook until it is very poached.
When the onion is golden brown, add the spinach. Sauté together with the onion. Then add the flour, stir everything together and continue cooking the flour for a minute.
Add the milk little by little and add the grated cheese. Continue stirring until the spinach and cheese croquettes come away from the pan.
Remove the dough from the pan and place it in a bowl. Once it has cooled, put it in the fridge for 3-4 hours.
Beat the eggs in a bowl and put the breadcrumbs in another bowl. Form the cheese and spinach croquettes. Dip them in the egg and breadcrumbs.
Fry the croquettes in a frying pan with plenty of hot oil. Brown them and take them out. Until they are all ready.
Serve the dish as a starter, appetizer or tapas.