Make the stock up 400ml/1½ cups boiling water to 1 beef stock cube.
Mist a large frying pan with low calorie cooking spray, then heat over a medium heat.
Add the meatballs and cook until they have browned.
Remove the meatballs from the pan, return the pan to the heat and spray with a bit more low calorie cooking spray.
Add the chopped onion and mushrooms and cook until they start to brown.
Add the herbs, balsamic, Worcestershire sauce and seasoning and cook for a minute.
Pour in the stock, bring to the boil and simmer for 4 or 5 minutes.
Place half of the sauce into a jug and whisk in the xanthan gum a ¼ of a tsp at a time. You will need to whisk well - and keep whisking!
Add the meatballs back into the pan and cook in the oven for 20 minutes at 180°C.