Combine 2 tbsp oil and salt and pepper in a medium bowl. Add mushrooms and onions and toss to coat in oil mixture.
Preheat a greased barbecue grill and plate on medium-high heat.
Brush both sides of steaks with 1 tbsp oil and season with salt and pepper. Place steaks onto barbecue grill and cook for 3-4 minutes (for medium) on each side or until cooked to your liking. Remove to a plate, cover with foil and set aside.
Meanwhile, place mushroom and onions onto barbecue plate and cook, tossing frequently, for 2-3 minutes or until tender. Remove to a plate.
Place tomatoes onto barbecue grill and cook for 1 minute on each side or until warmed through.
Slice steaks across the grain into thin slices. Place steak, mushrooms and onions and tomatoes into a heatproof bowl and toss gently to combine. Toss through rocket leaves and arrange onto serving plates. Drizzle salads with remaining oil, season with salt and pepper and serve.