Melt the butter in a large non-stick skillet. Cook mushrooms over medium heat for 5-10 minutes, or until soft. Sprinkle flour over the mushrooms, and stir to combine until all the lumps are gone. Add the vegetable stock, cream, and seasonings. Stir and reduce heat to medium-low. Simmer until thickened.
Once thickened, remove from heat and let cool.
Notes
If you want to eat this as soup right away - add more liquids. As written this will make a very thick, condensed soup.