Video: Christmas leftovers: how to create a Spanish tortilla from leftover roast potatoes 1) Saute the onions for about 10 minutes until they are soft. Add the garlic and fry for another 2 minutes. 2) Add the onions to the potatoes and allow to cool. Then add the eggs and mix. 3) Heat a small deep 6-8 inch frying pan with a good splash of light olive oil. 4) When the oil begins to smoke, pour the egg mixture into the pan. Be careful, it may spit. 5) As the mixture enters the pan, swivel it from side to side using the handle so the egg doesn't stick. You should see the egg moving independently from the pan itself. Turn the heat down low, to stop it burning. 6) Cook for 4-5 minutes. Place your finger in the middle and feel where the egg is cooked too. When it is cooked halfway through it is ready to turn. 7) So this is the fun bit. Place a plate on top of the tortilla and flip onto the plate. 8) Return the pan to the heat. Scrape off any bits that may have stuck and add a little more oil. When hot, slide the tortilla back into the pan, raw side down. Shake from side to side again to make sure its not stuck. Turn the heat down. 9) Using a wooden spoon, tuck the rough edges underneath to make them rounded. Cook for a further 5 minutes. Slide onto a plate and allow to cool.