In a large pot, bring the chicken stock, soy sauce & sherry to a simmer.
Meanwhile, in a frying pan, saut� the onion & garlic in the butter until they are soft, add the mushrooms.
When all of the veggies are tender, add them to the stock in the large pot, including all the liquid expressed during cooking. Add the black pepper to taste & simmer for 20 minutes at very low heat.