In a frying pan, take some oil and heat it. Add chopped potatoes, largely cut onion pieces and pinch of salt into heating oil.
Cook for 10 minute while stir it with some wooden spoon. Potatoes will become soft and you will crush them to make like a paste. When all potatoes are crushed into paste, then cook for more 5 minute and remove from heat.
In separate bowl, pour 5 eggs, little bit of salt and beat them together. Then add potatoes mixture into this smashed eggs and mix both ingredients.
Now in a deep pan, put some olive oil on medium heat and pour 1/3 of all potatoes/egg mixture into it. Spread it evenly and place cheese slices along with chicken slices over it. Cover these cheese/chicken slices with remaining potatoes/egg mixture.
Cook it for next 5 minutes at low heat. After 5 minutes, place some dish over it and flip frying pan so that cooked side should come at top. Slip/put back omelette in frying pan to cook other side. Cook for another 5 minutes and your tasty Spanish potato omelette sandwich is ready.
Adapted from contejo.com