The Spanish potato tortilla needs no introduction - with onion or without, more or less set, warm or at room temperature. But precisely because it is so well known, it can take some play. For years, Spanish bars and taverns kept stuffed and topped tortillas in their display cases, and now they are making a comeback. The base is the same for all three versions: potatoes, eggs, onion and good olive oil. Everything else is an upgrade.

The base. For a 24-centimetre pan and 4 to 6 main-course portions you need 700 grams of potatoes, half an onion, 7 large eggs plus one yolk, around 500 millilitres of olive oil and salt. Cut the potatoes into even 2-3 millimetre slices and the onion into thin strips. Put them into the heated oil with a little salt and cook over medium heat for 20-25 minutes, stirring carefully, until the potato is completely soft but has barely any colour. That is the secret: the potato isn't fried brown, it is slowly poached in the oil. Then drain for a few minutes, and strain the oil and keep it for something else.

Lightly beat the eggs with the yolk and a little salt, add the potatoes and let the mixture rest for 5 minutes. In a non-stick pan with a spoonful of the saved oil, set the tortilla over medium heat for about 3 minutes, until the edges firm up, then flip it with the help of a plate and cook for another 2-3 minutes on the other side. If it is getting a cold filling, the centre must be set and the tortilla completely cooled before stuffing.

With bacon, cheddar and mozzarella. Heat the oven grill to 220 degrees. Fry 120 grams of bacon cut into strips in a dry pan for 4-5 minutes and drain on paper. Put the freshly cooked tortilla in an ovenproof dish, scatter 100 grams each of grated cheddar and mozzarella over it, then the bacon, and gratinate for 3-4 minutes, until the cheese melts and starts to colour. Serve immediately, with a little freshly ground black pepper. The short gratin matters: the cheese melts, and the tortilla doesn't lose its creamy centre.

With tuna and roasted peppers. This is the version closest to the one in the old display cases. Crumble 250 grams of well-drained tuna in olive oil and mix it with 5-6 finely chopped roasted piquillo peppers, 150 grams of mayonnaise made with pasteurised eggs and a little pepper. Spread the filling over the cooled tortilla, leaving about a centimetre of free edge. From two eggs beaten with salt, in the same 24-centimetre pan with a teaspoon of oil, make a thin round omelette and, while it is set but still pliable, lay it over the filling like a lid. Keep in the fridge until serving.

With lettuce and tomato. The freshest variation: spread half of 120 grams of mayonnaise on the cooled tortilla, top with 6 inner leaves of romaine lettuce cut into thin strips and two medium tomatoes in slices, a little salt, then the rest of the mayonnaise. On top goes another thin two-egg omelette. The lettuce must be well dried and the tomatoes ripe but firm, otherwise the water ruins the filling. The recipe calls this "hat" a txapela, like the Basque beret.

Tortillas with a cold filling can be made a few hours ahead, kept covered in the fridge and should be eaten within 24 hours; take them out 10-15 minutes before serving. The lettuce one is assembled just before eating, and the gratinated one is best straight away. Every Balkan kitchen knows potatoes and eggs in a pan - but few ever thought of giving it a hat.