Step 1Combine wasabi, mirin, soy sauce and vinegar in a small bowl. Place half of the marinade in a shallow glass or ceramic dish; reserve the remaining marinade. Add steak to dish and turn to coat. Cover and set aside for 30 minutes.
Step 2Preheat a chargrill pan or barbecue hotplate to medium heat. Drain the steak of excess marinade; lightly spray with olive oil. Grill for 2–3 minutes each side, for medium, or until cooked to your liking. Transfer to a plate, loosely cover with foil and set aside for 5 minutes. Slice thinly.
Step 3Meanwhile, cook soba noodles in a saucepan of boiling water according to packet instructions, or until al dente. Add broccolini for last 2 minutes of cooking time. Drain well.
Step 4Combine noodles, broccolini, spinach, capsicum, onion and beef in a large bowl. Add the reserved marinade and gently toss to combine. Garnish with sesame seeds to serve.