Combine the oil, balsamic vinegar, garlic, golden syrup and sweet chilli sauce in a screw-top jar. Season with salt and pepper. Shake until well combined.
Place the mushrooms in a large ceramic bowl. Pour over the dressing and stir to coat. Cover and stand for 30 minutes to marinate.
Preheat oven to 220°C. Spoon the mushrooms and any remaining marinade into a greased baking dish or roasting pan. Roast for 20–25 minutes, tossing twice during cooking, or until tender. Sprinkle with herbs. Serve.