I made my life easy by slicing the potatoes and onions using a small food processor. When the oil was hot I poured it all in the pan and after a few second I turned the heat down a little so let the potatoes and onions "stew" not fry crisp. When cooked I carefully removed the potatoes and onions from the oil and poured them in the bowl of beaten egg. I mixed everything well and I added some salt. Using a smaller non stick pan I poured some of the mixture onto the oiled pan. I only greased the pan with oil at this point. When the bottom side was cooked I removed the pan from the heat and placed a plate that was larger than the pan over it. With my hand firmly placed on the plate and the other on the frying pan handle I flipped the whole thing over leaving the tortilla cooked side up on the plate. The frying pan went back onto the heat and I eased/slid the omelette back onto the pan ucooked side down.
Tip: If like us you try and grow your own veg. then you know that courgettes and marrows grow in large quantities. I try to use these in many different ways just to eat them up. I have used these in our tortillas instead of potatoes. They worked well. Just remember to use the dry fleshy bits or it will be too soggy. They make great Tortillas. I peeled the marrows or courgettes and fried them using a non stick pan which was only greased with oil so this is great if you want a low calorie tortilla.