Soak the shallots in half the lemon juice. Brush the steak with oil and season well. Cook over a high heat for 2 minutes on each side. Leave to rest for 5 minutes then slice thinly. Catch any juices and add them to the remaining lemon juice with enough olive oil to make a vinaigrette.
Toss the rocket with the drained shallots and dress with the vinaigrette. Top with the beef.