Whisk soy, 1 tbs vinegar and wasabi in a jug until combined. Place steak and half the wasabi mixture in a bowl, turning to coat. Marinate for 20 minutes. Combine carrot with remaining vinegar in a bowl and set aside.
Heat a chargrill pan over medium-high heat and bring a saucepan of water to the boil. Cook noodles for 4 minutes or until tender. Drain and return to pan to keep warm.
Chargrill steak for 3 minutes on each side for medium-rare or until cooked to your liking. Transfer to a chopping board, cover with foil and rest for 5 minutes. Thinly slice.
Meanwhile, chargrill onion and broccolini for 4 minutes or until softened. Divide noodles among serving bowls. Top with carrot, spinach, onion, broccolini and steak. Spoon over remaining wasabi mixture, baby spinach and sprinkle over sesame seeds.