Place mushrooms and salad dressing into a bowl, toss to coat. Cover and set aside to marinate for 30 minutes. Cook rice and quinoa following packet directions. Stir with a fork to separate grains then set aside to cool to room temperature.
Preheat oven to 200°C (fan forced). Place capsicums and onions into a large greased roasting pan. Spoon over remaining salad dressing and season. Roast 15 minutes or until just tender. Add tomatoes and olives. Roast a further 5-8 minutes or until just warmed through. Set aside 5 minutes.
Add rice and quinoa, mushrooms and parsley to roast vegetables, toss gently to combine. Drizzle with extra virgin olive oil, season to taste, serve.