Place mushrooms into a ceramic bowl. Mix the olive oil and vinegar together with seasoning and pour over the mushrooms. Toss to coat. Cover and stand 15 minutes to marinate.
Preheat a barbecue plate on high until hot. Reduce heat to medium. Brush both sides steaks with oil. Barbecue 4 to 5 minutes each side for medium or until cooked to your liking. Transfer to a plate and cover with foil to keep warm.
Drain the mushrooms and add to the barbecue plate. Cook, tossing for 3 minutes until just tender. Transfer to a bowl. Add tomatoes and stir to combine.
Place steak onto plates. Spoon over mushrooms and serve immediately.