1. Fry mushrooms in butter until limp and cooked through
2. Make a roux by adding 1/4 cup tapioca or potato starch to the mushrooms, over the heat and stirring well to completely coat the tapioca starch with butter and mushrooms, and toast it lightly
3. Because the pan is hot you will stir in cold liquid.  Using a whisk, stir in 2 cups of cold water, slowly adding the water
4. The liquid will begin to thicken immediately.  Keep whisking so it doesn’t form lumps
5. Simmer for a minute to meld flavours and adjust the thickness, by adding additional liquid, according to your taste
6. Add salt and pepper to taste

For a richer soup, you can add 1 cup of cold water and 1 cup of cold cream or whole milk or buttermilk.  If the mixture gets too thick, thin with a little milk, 1/4 cup at a time till it’s the consistency you need for your recipe