I large potato
80g grated cheese, such as Gruyere or Cheddar
Peel the potato, cut it into equal-sized pieces, put them into cold water, bring to a simmer, and cook until soft. Drain. Then -- this is the different bit -- mash them.
Meanwhile, beat the eggs lightly. Mix them with the cheese and potato, and a little salt if you like.
Melt a large knob of butter in a heavy, 28cm frying pan over the lowest possible heat. Pour in the egg mixture. Cook until there is a decent amount of set egg on the bottom -- about 10 minutes. Put the pan under a low grill for a minute or so, to set the top.
I got this idea from Allegra McEvedy's column in the Guardian. I am sure that her spicy version would be delicious. I am less sure about baking it -- the egg would be more likely to toughen, I think. I certainly would not entrust the mixture to my Pyrex dish: it would stick. However, one of the effects of the mash seems to be to give a more tender result.
My tortilla would have benefited from a couple of sliced Spanish onions, fried very gently and tipped into the egg mixture before being returned to the pan.