Peel the potatoes and cut them in half. Place the potatoes cut side down and thinly slice. (SEE NOTE 2)
Peel onion, cut in half and thinly slice
Heat 2tbsp of the olive oil in a non-stick frying pan, over medium heat. Add the potato and onion slices, reduce heat to low and cover. Cook for 20-30 mins, until the potato slices, have cooked through, giving them a gentle stir every 10 mins.
Crack the eggs into a large mixing bowl, season and whisk. (do not add salt if cooking for baby / young child). Once the potatoes and onion have finished cooking add them to the eggs. (SEE NOTE 3)
Add the remaining 1tbsp of oil to the same pan used to cook the onions and then pour the egg mixture into the pan. Cook over medium heat for around 3 minutes, or until the bottom of the omelette is lightly golden, set and coming away from the sides. The top of the tortilla will still be a bit wet, which is fine.
Carefully place a large plate over the top of the pan (SEE NOTE 4), then flip the pan and the plate together. If the bottom is properly cooked then the tortilla should end up on the plate in one piece (with a little bit of runny egg leaking out)
Return the pan to the heat and slide the tortilla back into your pan to cook the second side, cook for a further 2-3 mins until the bottom is also lightly golden.