Heat 2 tablespoons of the oil in a large frypan. Add the onions and cook for about 8 minutes, until they are pale golden.⏱ 8 min
In a large bowl, whisk the eggs with the salt and pepper. Cut cooled potatoes into 1/2cm slices and stir into the egg with the onions and parsley, making sure all of the potato slices are separated and coated with egg.
Heat remaining oil in frypan used for onions. Add egg mixture. As the base begins to cook, use a spatula to lift it in several places, allowing the raw egg to run underneath. Once it starts to set, leave it to cook over a very low heat until just set through (about 12 minutes).⏱ 12 min
Run a knife around the edge to loosen, flip onto a plate then slide back into the pan to cook through. Test by gently pressing the centre - if it bounces back it's cooked. Cool for 5 minutes before turning out onto a plate.⏱ 5 min