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Cut the potatoes into slices, about finger thickness. Heat the oil in a non-stick frying pan, then tip in the potatoes and onion. When they start to sizzle and colour a little, pour in 225ml/8fl oz water - this will splatter so be careful. Simmer potatoes for 10 mins until tender.
Pour off any excess liquid from the potatoes, then tip the eggs into the pan, season and stir lightly to mix. Return the pan to a medium heat and cook the tortilla until set on one side, about 5 mins. Place a plate on the top of the pan, hold it tightly and deftly turn it upside down so the tortilla comes out onto the plate. Add a bit of extra oil to the pan if it needs it, before sliding the tortilla back in to cook the reverse side - it only needs 5 mins more.
Slide the tortilla onto a plate and cut it into 4 wedges. Top with a little pile of rocket, sprinkle over the parmesan, then drizzle with oil and serve.