Blanch the green beans and butter beans in boiling water in a medium saucepan for 1 minute and then cool under cold running water. Toss with the tomatoes, artichoke hearts, lettuce and rocket leaves in a salad bowl and refrigerate until ready to serve.
Preheat a ridged, iron grill pan. Cook the steaks for 3 minutes each side or until done to your liking. Baste on both sides with the mustard towards the end of the cooking time.
Toss the salad with the dressing just before serving and arrange on four plates. Serve with the beef medallions alongside.