Heat the olive oil in a large, nonstick frying pan. Add the onion and potato and cook for 5–10 minutes until golden, then pour in the measurement water. Simmer gently until the potatoes are very tender, then pour away any excess liquid.
Season the eggs well, then pour into the pan and stir gently. Cook over a gentle heat for 10–15 minutes until set all the way through, finishing off under a preheated grill to set the top if necessary.
Toss the rocket with the extra virgin olive oil, lemon juice and tomatoes. Season well and add the Parmesan shavings. Cut the tortilla into wedges and serve topped with the salad.