Cover and chill for at least 1 hour. Set aside remaining marinade.
Meanwhile, blanch potato in boiling, salted water for 5 minutes or until just starting to cook on the outside but still firm inside. Drain and refresh, then place in a bowl with onion and reserved marinade, turning gently to coat. Cover and set aside for 20 minutes.
Heat an oiled chargrill or barbecue over medium heat. When hot, cook the potato and onion, basting frequently with marinade and turning once, for about 15 minutes or until lightly charred all over. Keep warm. Increase the heat to high and re-oil. When hot, cook beef for about 6-7 minutes each side until medium-rare (or until cooked to your liking). Remove from the heat and stand for 5 minutes, then slice.
Divide the beef, potato and onion among serving plates. Sprinkle with the rocket leaves and serve with lemon wedges.
Serving size: 4
Recipe courtesy of delicious.com.au