In a large saucepan over medium heat, melt the butter and sweat the onion and garlic for about 3 to 4 minutes. At this point, raise the heat to high, add the wine, and boil 10 minutes, until reduced to dry. Then reduce the heat to medium, add the mushrooms, cook until soft, and add the stock. Then, raise the heat to be high and bring the mixture to a boil. Reduce the heat to medium and simmer for 15 minutes.