Peel the potatoes and cook in salted water until almost tender using the point of a knife to check for softness.
Meanwhile heat the olive oil in a medium non-stick frying pan, add the onions and cook over a low heat for about 15 minutes. They should be soft, sweet and golden. Use a little water to help soften them, it will evaporate and help to cook the onions slowly. Add the chorizo and cook until pale golden. Drain and reserve the oil.
Break the eggs into a bowl, whisk lightly and add ¼ teaspoon of pepper, the cooked potatoes, onions, and chorizo, mix well.
Heat the pan with 1 tablespoon of olive oil, wait until really hot, then pour in the mixture, shaking the pan with the other hand. Reduce the heat to low, and cook for 3-5 minutes until the underside is golden brown.
Remove from the heat, place a plate over the pan, then using 2 cloths take each side of the pan and invert the tortilla.
Place the pan back on the heat, add a little more olive oil, heat until smoking, then carefully slide the tortilla back in the pan to cook the other side. Pour over the sherry and flame.
Turn out onto a heat proof serving plate, lavish with Serrano ham and manchego cheese and flash under a hot grill until molten and golden.
When firm to the touch in the middle, slide onto a serving plate, rest for 5 minutes before cutting.
