For the Soup, place all ingredients in blender container; cover. Blend on high speed until smooth. Remove insert from blender cover. With blender on, gradually add enough water (1 to 1 1/2 cups) until soup is a drinkable consistency.
For the Topping, heat oil in medium nonstick skillet on medium heat. Add remaining ingredients; cook 5 to 7 minutes or until mushrooms begin to brown and pistachios are toasted, stirring occasionally.
Pour soup into beverage glasses. Top each with 1 tablespoon of the Topping. Add additional Topping, if desired.