1.Combine horseradish and oil in large bowl; add beef, turn to coat in mixture. Cover; refrigerate 3 hours or overnight.
2.Cook beef in heated oiled grill pan, turning, until browned. Reduce heat; cook beef, turning occasionally, about 30 minutes or until cooked. Cover; stand 10 minutes.
3.Meanwhile, make rocket salad by placing ingredients in large bowl and tossing gently to combine.
4.Preheat grill (broiler).
5.Combine mustard, parsley and breadcrumbs in small bowl with half the butter. Brush beef with remaining butter; press breadcrumb mixture over beef. Grill beef until crust is browned. Stand 10 minutes; slice thickly.
6.Serve beef with rocket salad. Place ingredients in large bowl; toss genty to combine.
Horseradish is sold in various forms; make sure you use the prepared white horseradish version in this recipe and not horseradish cream.Note