You cut into a thick gold round and the centre should still tremble — a dry tortilla is a failure of nerve, and you will know it at once. Onion or none is a civil matter you will settle for yourself; the only consensus is that it should be thick enough to slice.
It is a national dish with no single birthplace. Potatoes came from the Americas; the rest is Spanish kitchen sense — eggs, oil in quantity, a pan you can invert. Extremadura and Navarre both claim an origin; what travelled was the method, not a recipe.
Serve it warm or at room temperature, in wedges, as a tapa or a meal. A still-juicy centre is not undercooking. You want a Verdejo or a Basque Txakoli beside it, and nothing else.
Method
- 01
Slice the potatoes a scant ⅛ inch thick — thin enough to poach in oil without falling apart. Warm the cup of olive oil in a deep 10-inch pan; you are poaching, not frying, so keep the heat medium-low. Add the potatoes and sliced onion and cook, stirring now and then, until the potatoes are tender but not browned, 15 to 20 minutes. They should yield to a fork and smell sweetly of onion.
- 02
Drain the potatoes and onion in a colander set over a bowl, and do save that oil — you'll need it. Season the potatoes well with salt while they're still warm, so it goes in. Beat the eggs with salt in a large bowl and fold in the warm potatoes and onion; you want everything coated, not mashed. Let the mixture rest 5 minutes so the eggs begin to settle around the potato.
- 03
Return 2 tablespoons of the reserved oil to the pan and set it over medium heat until it shimmers. Pour in the egg and potato mixture and give the pan a shake to settle it into an even round. Cook until the edges are set and the underside is a confident gold, 4 to 5 minutes. Don't fuss with it; a tortilla needs to find its shape.
- 04
Now the famous flip: invert a plate over the pan, hold it firmly, and turn the whole thing over in one decisive motion. Slide the tortilla back into the pan and cook 3 minutes more for a still-juicy center — a dry tortilla is a failure of nerve. Rest it a minute so it finishes setting, then slice into wedges. Serve warm or at room temperature; both are correct.
