1. Put a large non-stick frying pan on a low heat.
Cook the onion slowly in the oil until soft.
2. Add the sliced potatoes to the pan and cook, stirring occasionally to make sure they fry evenly. When the potatoes are soft and the onion is shiny, remove most of the oil, crush 2 garlic cloves and stir in, followed by the beaten eggs.
3. Put the lid back on the pan and leave the tortilla to cook gently. After 20 mins, the edges and base should be golden, the top set but the middle still a little wobbly. To turn it over, slide it onto a plate and put another plate on top, turn the whole thing over and slide it back into the pan to finish cooking.
4. Once cooked, transfer to a plate and serve the tortilla warm or cold, scattered with the chopped parsley.