use only the freshest organic free-range eggs. That way you can keep the centre of the tortilla quite moist without worrying about getting sick
peel the potatoes and onions, cut each of them in half or into four pieces, depending on size, then cut each piece into 1/2cm slices. Keep the onion and potato slices separate
sprinkle a little salt on the potatoes and let stand while you heat up the oil. You want enough oil to cover the potatoes. The oil is ready when a slice of potato just sizzles. Turn up the heat and add the rest of the potatoes a handful at a time, waiting for them to sizzle before adding the next batch
when all the potatoes are in the pan turn down the heat to medium and cook without browning for 3-4 minutes. Now add the onions. Continue cooking until the potatoes are soft and the onions translucent.
lightly beat the eggs and add the finely chopped garlic,salt, pepper and mace. Add the cooked potatoes and onions, adjust the seasoning and mix well.
heat a heavy based frying pan for half a minute or so then put in some of the still hot frying oil. Add a small drop of egg. If nothing happens the pan is too cold and the tortilla will stick. If the egg explodes and spits it's too hot and the tortilla will burn.
add the tortilla mixture to the pan. After a minute or so ease the base of the tortilla free with a flexible spatula. Shake the pan form side to side until your tortilla slides freely. Cook for just a couple of minutes, shaking occasionally until the egg has set to about half to two-thirds of the way
to turn your tortilla place a plate on top, flip it and slide the tortilla back into the pan - after adding a touch more oil
cook the tortilla for a further couple of minutes.