Remove the potatoes and the onions from the oil. Discard the oil (I save about 1 tbsp for cooking the omelette, I just let a bit on the pan).
In the same nonstick pan add the egg and potatoes mix (i let a bit of oil). Cook over medium heat for 6-8 minutes until the omelette is 70%-80% done.
Flip the omelette over a flat plate. Then slide the tortilla that is flipped in the plate back to the pan. Cook for 2 more minutes and it’s done.