My way of making potato tortilla (tortilla de papas, in Argentina) is frying potato slices until crispy. Then submerging in the bowl where the beaten eggs are and leave the fried potatoes to soak and become soft. I add generous amount of grated Parmesano or Sardo cheese. We use in Argentina a double frying pan with teflon (about 20 cm diameter, they have a hinge in the front and folds one on top the other.) That makes easy the flipping over of the tortilla: no danger of spilling egg all over the floor of sending the tortilla to oblivion.
The pans are covered with melted butter and I make the tortillas rather thin, about half the thickness of the ones seen in the photos. I cook it about 2 minutes on one side but make sure there are lots of liquid egg left before turning the frying pans. I turn the tortilla over and give another minute or two over a moderate the fire, that way the torilla comes out "juicy", with uncooked egg flowing from the tortilla. Spicing is salt, pepper and absolutely no onions!