Heat the olive oil in a pan and add the onion. Cook very slowly with a pinch of salt until deep gold - about 30 minutes - stirring every so often. Drain, reserving the oil, then pour the oil back into the pan and add the peppers and garlic. Fry for 5-10 minutes until completely tender. Whisk together the eggs and a little seasoning in a large bowl, then add the onions, peppers and garlic, again reserving the oil. Put a large frying pan on the heat and add the reserved oil. When nearly smoking, carefully pour in the tortilla mixture, shaking the pan slightly to ensure it is evenly spread. Reduce the heat and cook for about five minutes until the underside is golden-brown. Place a similar-sized plate on top of the pan and carefully, using a cloth, invert the tortilla on to the plate. Add a little extra oil into the pan then slide the tortilla back into the pan and cook for another 3-5 minutes, until the centre is set. Slide back on to the plate and leave to cool for a few minutes.