Wash and peel the potatoes, then sliced them using a mandolin for thinner slices or a sharp knife for thicker ones. Finely chop the onion (I sliced it).
Heat the olive oil in a pan and cook potatoes and onion, continuously stirring, until potatoes are cooked but not too tender.
Remove the excess oil and keep aside.
In a bowl, beat the eggs with salt and pepper. Transfer the cooled down potatoes in this bowl. Mix well add rosemary.
Warm a couple of tablespoons of olive oil in the frying pan and cook the mixture potatoes/eggs for 5-8 minutes or until golden. Do not stir. Then with the help of a flat plate flip the tortilla of potatoes and cook the other side.
Serve warm or cold, cut into slices or cubes, together with a salad.