Wrap the beef in cling film; freeze for about 1 hour until firm. Cut the beef across the grain into 5mm/1⁄8in slices. Put the slices between two sheets of cling film, and pound with a mallet or rolling pin until paper thin. Roll the beef up in cling film once again, and chill for 1 hour.
To make the vinaigrette, whiz the lemon juice, vinegar, oil and salt in a blender until smooth. Pour into a small bowl, and stir in the capers.
Arrange the beef slices in a single layer on one large serving plate or six individual plates. Toss the rocket with half the dressing. Pile the rocket on top of the beef. Top with the Parmesan, and serve accompanied by the remaining dressing.