Preheat your grill to high.
Cut the potatoes into thickish slices using a sharp knife.
Gently whisk the eggs and season with salt and pepper and set aside.
Place a large non-stick oven-proof frying pan over medium heat and add 3 tablespoons of oil.
Add the potatoes, tossing from time to time, for approximately 15 minutes. Add the onion and cook for five minutes, stirring often. Add the garlic and cook for a further 30 seconds or until cooked and aromatic.
Add the sherry vinegar and sugar to the pan and stir for a minute or two. Remove from the heat and gently mix the potato and onion mixture into the eggs.
Heat the reserved 1 tablespoon of oil in the same frying pan over medium heat. Add the egg, potato and onion mixture and as it starts to cook, gently agitate the pan. After 1-2 minutes, cover the pan with a large oiled dinner plate, flip the pan and turn the tortilla out onto the plate.
*** If you are not keen or confident to do this above step, please read below for an easier version.
Gently slide the tortilla back into the pan but this time with the uncooked side down. Turn heat down to low.
After a minute, transfer the tortilla to under the hot grill and cook for approximately 2-3 minutes or until the top is set. It is fine for the tortilla to still have a little wobble to it. Keep in the pan for approximately 5 minutes before serving.
Serve warm or at room temperature.
*** If you would rather not flip the tortilla, you will still get an excellent result but cooking it in the pan for five minutes longer without flipping. Place the frying pan in the oven or under the grill for approximately 2-3 minutes. Your omelette will then be set enough so that you can confidently place a plate (larger then your pan) over your pan and carefully invert the Spanish potato omelette onto the plate.