Heat the oil and a large nob of butter in a large frying pan, mix the onions and potatoes well and then add them to the pan and stew gently, partially covered, for roughly 10 minutes. Add the rest of the foraged leaves and leave for another 15 min, stirring occasionally until the potatoes are softened.
Carefully pour the potatoes and veg through a colander into a large bowl and pour the oil back into the pan. Beat the eggs in another bowl and then pour onto the potatoes and veg with plenty of salt and pepper and stir it all together, then let it sit a while whilst you re-heat the pan.
Tip everything into the pan and cook on a gentle heat for about 5 min without doing anything, then use a spatula to shape the omelette into a cushion by softly pulling the edges in and slowly rotating the pan to flood the empty space.
When it’s almost set with a little liquid still visible, take the pan off the heat and leave for 4 min to cool a little, then place a large dinner plate on top and carefully flip the pan so that the tortilla is now on the plate, then slide it back into the pan and cook for a few more minutes. Invert twice more, cooking the omelette briefly each time and pressing the edges to keep the cushion shape. Slide it back onto a plate and cool for 10 minutes before serving. Eat and of course, enjoy!