Prepare the salt cod. Alternatively, make your own salt cod (below). Using your fingers, break up into flakes.

Heat 80 ml of the oil in the frying pan, add the onion and potatoes and fry for about 10 minutes until softened but not coloured - turn them frequently to prevent sticking.

Lightly beat the eggs in a bowl with pepper. It usually isn't necessary to add salt because the fish is already salty. Mix in the spinach and flaked fish.

Pour the mixture into the pan, moving it with a spatula so it flows under and over the potatoes. Cook until set on the bottom - shake the pan as it cooks and loosen the sides a little with a spatula. When it has set and is pale golden, put a plate on top of the pan, turn it upside down, then slide the tortilla onto the plate.

Put the remaining oil in the pan. When hot, slide the tortilla back into the pan, cooked side up. Cook until golden. Slide onto a board or plate and leave for a minute before slicing.

Note To make your own salt cod, sprinkle a non-reactive dish with 75 g rock salt and put a 200 g skinless cod fillet on top. Cover with another 75 g salt and chill for 12 hours. Wash off the salt and soak in water for about 2 hours, changing the water several times. This recipe produces an attractive white result - ordinary dried salt cod can be cream in colour.