Combine olive oil, lemon juice and zest in a bowl. Whisk together. Add all the vegetables to the bowl and stir to coat. Season well. Cover and refrigerate for 1 hour. Preheat oven to 200˚C. Season portabellos lightly with salt, pepper, chilli flakes & oregano.Roast on a rack on top of a baking tray for± 7 minutes until tender. Fill warm portabellos with a pile of the marinated mushroom mixture. Serve straight away.