Melt butter in a large pot. Sweat the garlic and shallots in the butter for 20 minutes, until translucent. Add mushrooms and sweat until mushrooms are soft.
Deglaze pot with white port, and cook until reduced by half. Add the water; cover. Bring to a boil, then remove from the heat and allow to steep for 20 minutes. Add salt and pepper to taste.
To serve, ladle bouillon into a soup bowl, and top with chives. Finish by floating several dashes of sherry on the surface of the bouillon.