Prepare half a litre of vegetable stock with the stock cube and set aside.
Add the oil to a frying pan and sauté the onion with the sugar for 10 minutes.
Once it's caramelised, add the arrowroot powder to the pan and stir for a few minutes to combine.
Pour in the wine, balsamic vinegar and tamari sauce and simmer until the stock is reduced by half.
Add the vegetable stock and simmer again for 10 minutes until you are left with a thick, dark gravy.
Optional: for a smooth gravy, blend for a few minutes until no lumps remain.