Spanish Potato Tortilla

Break the potato slices apart with your fingers so none of them are stuck together.

Pour the olive oil into a 20cm frying pan, which should be deep enough to cover the potatoes once they go in.

Warm the oil over medium-low heat to about 130C/265F, where a slice dropped in sits still and gives off a slow stream of small bubbles rather than sizzling hard.

Slide in the potatoes and the sliced onion.

Cook for 20-25 minutes, moving them gently with a slotted spoon every few minutes, until a slice crushes against the side of the pan with no resistance in the middle.

Keep the oil below a fry the whole time. This is a poach in oil rather than a chip pan, and browned potatoes stay firm and taste of chips where what you want is a slice soft enough to collapse into the egg.

Tip the potatoes into a colander set over a bowl and leave them to drain for 5 minutes.

Keep the drained oil. It has taken up the flavour of the potato and onion and will do the next tortilla or a pan of fried eggs.

Beat the eggs with the 6g of fine salt in a large bowl until the colour is even.

Fold the hot potatoes and onion into the eggs.

Stand the mixture for 10 minutes, stirring once, until the potatoes have drunk some of the egg and the whole thing has thickened. Skip the stand and you get a tortilla in two layers, egg around potato, rather than one thing.

Wipe the pan out and set it over medium heat with 2 tbsp of the reserved oil.

Pour the mixture in once the oil shimmers.

Stir it three or four times in the first 30 seconds, which sets the egg evenly through the potato.

Stop stirring after that and draw the setting edge away from the side of the pan with a spatula, tilting the pan so the loose egg runs underneath.

Cook for 4-5 minutes, until the sides are set, the base is pale gold when you lift an edge and only the middle still moves.

Take the pan off the heat.

Set a plate wider than the pan face down over it.

Hold the plate flat with one hand and the handle with the other, and turn the whole thing over in one committed movement, away from you.

Do it in one go. A slow, hesitant flip is how a tortilla ends up on the floor, and hot oil comes over the rim if the plate is not held flat against the pan.

Slide the tortilla back into the pan, cooked side up.

Tuck the edge under all the way round with the spatula, which is what gives it its cushion shape.

Cook for a further 3-4 minutes, until the second side is gold and the tortilla slides freely when you shake the pan.

Press the middle with a fingertip: it should give like a set custard rather than spring back. A tortilla cooked firm to the centre is dry, and juicy is the whole argument in Spain.

Slide it onto a board and rest it for 10 minutes.

Finish with flaky salt.

Cut it into wedges and serve warm or at room temperature, never straight from the fridge.