Preheat the grill to high.
Place all the ingredients in a sandwich bag and mix by squishing them around the bag.
Leave to marinate for 10 minutes.
Thread the mushrooms and onions onto a metal skewer and place on a baking tray.
Grill for 15 minutes, turning regularly. Remove from the grill and slice down the skewer to serve.
This recipe has been specifically designed for a low protein diet. Refer to labels for allergen and other product information.