Heat the butter in a medium saucepan and fry the onion over a low heat for 2 minutes, until softened. Add the mushrooms, garlic, vinegar and soy sauce and cook for 15 minutes.
Sprinkle the flour into the pan and stir until the flour has been incorporated and made a thick paste. Add the stock and bring to a boil, then lower the heat and simmer for a further 10–1