Place mushrooms into a ceramic bowl. Combine olive oil, lemon juice, garlic, sweet chilli, rice syrup, salt and pepper in a screw-top jar. Shake well to combine. Pour over the mushrooms, stir to coat the mushrooms. Cover and set aside for 30 minutes to marinate.
Preheat a barbecue plate on high heat. Add mushrooms and cook, tossing, for 4 to 5 minutes until tender and golden. Transfer to a plate. Cool 10 minutes.
Place prawns, rocket, cucumber, tomatoes and avocado into large bowl. Add mushrooms and their juice. Season with salt and pepper. Toss gently to combine. Serve.