Slice and sauté mushrooms and leeks in butter until tender, approximately 5 minutes. Gradually stir in flour, coating the mushroom/leek mixture. Add 26 ounces of the chicken broth, salt, & ground pepper and stir. Heat to a boil and then reduce, cover and simmer for 20 minutes.
With a hand held stick blender, blend the mixture until smooth. (The soup may also be blended in a traditional blender and then returned to the pot for finishing). Add the remaining chicken broth and heavy cream. Heat thoroughly.
Garnish with fresh chopped parsley and, if desired, whipped cream.
Note: If fresh leeks are not available at your market, Knorr’s Leek Mix may be used as a substitute.