Preheat a barbecue or char-grill on medium-high heat. Spray steak and mushrooms with oil and season with salt and 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, turning often until tender. Transfer to a plate.
Meanwhile, place noodles in a heat-proof bowl. Cover with boiling water. Set aside for 5 minutes. Drain, refresh the noodles in cold water then drain well again. Separate noodles. 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.