Preheat a barbecue or char-grill on medium-high heat.
Brush steak and mushrooms with olive oil and season with pepper. Barbecue or char-grill steak for 4 minutes on each side (for medium) or until cooked to your liking.
Remove to a plate, cover with foil. Set aside to rest.
Barbecue or char-grill mushrooms for 5 minutes, tossing often until tender. Transfer to a plate.
Meanwhile, cook the noodles in boiling water until tender (approx 5minutes). Drain and rinse in cold water to wash any sticky starch from them. Place noodles in a large bowl.
Combine kecap manis, sweet chilli sauce and oil in screw-top jar. Shake well.
Slice steak into 1 cm-thick slices. Add to the noodles with mushrooms, green onions, peanuts and dressing. Gently toss to combine. Serve.