For the salad, place the spinach into a bowl and add the lemon juice and zest, walnuts, onion and olive oil and toss well to coat. Season, to taste, with salt and freshly ground black pepper.
For the string potatoes, half-fill a deep-based pan with the vegetable oil and heat until a cube of bread dropped in sizzles and turns golden-brown in 30 seconds. (CAUTION: hot oil can be dangerous. Do not leave unattended.). Deep-fry the potato for 2-3 minutes, or until golden-brown and crisp. Remove with a slotted spoon and drain on kitchen paper.
Arrange the salad on a serving plate and lay the strips of steak on top. Place the string potatoes on top and serve.