The secret of tortilla is:

 the potato should be cut into strips very finnes, so it is flexible. Fry in olive oil without browning.  Filter and then add eggs, and salt. You can add the onion  in the end when you fry pototos.  Heat oil in pan and put the half of pototos put if you want fish, cheese or any thing you want and add the other half of pototos.  Cook until  that it is golden brown, cook the other side with the help of a plate plate. and enjoy in your meal. Bon app?tit