Thinly slice the onion halves and separate into half-moon shapes.
Peel and slice the potatoes into thin rounds and dry them with a tea cloth.
Heat 30ml of olive oil in a 20cm frying pan until smoking hot.
Add potatoes and onions to the pan and coat thoroughly in oil.
Season with salt and pepper, cover, and cook on the lowest heat for 20 minutes until tender.
Whisk the eggs lightly in a large bowl and add seasoning.
Transfer the cooked potato and onion mixture into the whisked eggs.
Add the remaining 15ml of oil to the pan over medium heat.
Pour the egg and potato mixture into the pan and immediately reduce heat to lowest setting.
Cook uncovered for 20 to 25 minutes, occasionally drawing in the edges with a palette knife.
Invert the omelette onto a flat lid or plate once the surface is barely liquid.
Slide the omelette back into the pan and cook for 2 more minutes.
Turn off the heat and let the omelette settle for 5 minutes before serving.