Heat the oil in a small frying pan and cook the onion over a low/medium heat with a pinch of salt. After a few minutes add the "cudols" or red pepper if using. Cook until soft.
In the meantime boil the potatoes until tender. Once tender, chop them up with a knife until you have fairly small pieces.
In a large metal or glass bowl whisk the eggs with a couple of pinches of salt, the amount you use depends on your taste.
Add the cooked chopped potatoes, onions and cudols or peppers if using to the whisked eggs. Stir to incorporate everything
Add about a teaspoon of oil to the pan you cooked the onion in and heat over a low to medium heat. Poor in the egg mixture
Cook for a few minutes until the centre isn't quite as runny and the bottom is browning, I use my sense of smell for this part!!
Place the plate on top of the frying pan and, keeping a firm pressure on it with your hand, turn over the frying pan so the omelette comes out on to the plate.
Slide the omelette back into the frying pan. Turn off the heat and allow to heat through until set, around 2 minutes. Serve.