Tortilla española (Spanish omelette) is the most important egg dish in Spanish cuisine and the technique is deceptively difficult to master. It's not a frittata (which is finished under a grill) or a French omelette (which is barely set). It's a thick, round cake of potatoes and eggs, cooked in olive oil, flipped using a plate, and finished so the outside is golden while the interior remains slightly runny (jugosa) or just set. The debate between runny and set centres is one of Spain's great culinary arguments. The potato must be confited — gently cooked in a generous quantity of olive oil — before the eggs are added.
Potatoes are peeled, sliced thin (3mm) or cut into small irregular pieces, and cooked in a deep pool of olive oil (not deep-fried — cooked gently at 130-140°C) until completely tender but NOT browned. This is a confit, not a fry. Onion (in the 'con cebolla' version — another great Spanish debate) is cooked with the potatoes until sweet and soft. The potatoes are drained (reserve that oil — it's now flavoured and precious), mixed into beaten eggs, seasoned, and left to sit 10 minutes so the potato absorbs egg. Cooked in a non-stick or well-seasoned pan in a thin layer of the reserved oil. The flip: a plate larger than the pan is placed over the pan, the whole thing is inverted, then the tortilla slides back in to cook the other side.
The flip is the moment of truth and requires confidence: plate on top, one firm motion, slide back in. Practice with a cold pan first if nervous. For the jugosa (runny) centre: cook 4-5 minutes first side, flip, 2 minutes second side, then let it rest on the plate for 5 minutes — carryover heat finishes the cooking. Serve at room temperature, never hot from the pan. The reserved potato-cooking oil is liquid gold — use it for vinaigrettes, to cook other vegetables, or for tomorrow's tortilla. A properly made tortilla with good olive oil, decent potatoes, and fresh eggs is one of the simplest and most satisfying dishes in the world.
Frying the potatoes at high heat — they should confit gently, not brown. Not enough olive oil during the potato stage. Not letting the potato-egg mixture rest before cooking — the absorption step is essential. Pan too hot during cooking — medium-low is correct. Flipping too early — the bottom must be set. Being timid with the flip — commit to it. Overcooking — the centre should be creamy, not dry.