Remove the center piece of the blender lid (to allow steam to escape); secure the blender lid on the blender. Place a clean towel over the opening in the lid (to avoid splatters). Process until smooth. Return the pureed soup to the slow cooker; stir in the vinegar. Ladle the soup … Treat dinner guests to this premeal soup loaded with earthy, umami flavor from the mushrooms and soy sauce. Puréeing only some of the slow-cooker mushroom soup gives the dish complex texture and eye appeal. Garnish with additional black pepper and chopped fresh thyme, if desired.