Soup 1 tablespoon extra-virgin olive oil, divided 4 cups sliced shiitake mushroom caps 4 cups sliced baby bella mushrooms 1 cup diced onion or shallot ½ cup diced celery 3 cloves garlic, minced ½ teaspoon dried thyme ½ teaspoon salt ½ cup dry sherry 4 cups "no-chicken" broth or mushroom broth 1 cup water 1½ cups chopped walnuts 2 teaspoons sherry vinegar ½ teaspoon ground pepper Mushroom-Walnut Topping 1 tablespoon extra-virgin olive oil 1 cup coarsely chopped shiitake mushroom caps 1 cup coarsely chopped baby bella mushrooms ½ cup chopped walnuts Pinch of salt 2 tablespoons sliced fresh chives
To prepare soup: Heat 1 tablespoon oil in a large pot over medium-high heat. Add 4 cups shiitake mushrooms, 4 cups baby bella mushrooms, onion (or shallot), celery, garlic, thyme and ½ teaspoon salt and cook, stirring occasionally, until tender, 7 to 10 minutes. Add sherry; increase heat to high and simmer, stirring often, until the sherry has evaporated, 2 to 3 minutes. Add broth and bring to a simmer. Puree water and 1½ cups walnuts in a blender until completely smooth, about 1 minute. Pour into the soup; return to a simmer. Reduce heat to medium-low and cook, stirring often, until the vegetables are very soft, about 5 minutes longer. Meanwhile, prepare topping: Heat oil in a medium skillet over medium heat. Add mushrooms and cook, stirring often, until soft, about 2 minutes. Add walnuts and salt. Cook, stirring occasionally, until hot, about 1 minute more. Puree the soup with an immersion blender or in a regular blender (in batches, if necessary) until very smooth. (Use caution when blending hot liquids.) Stir in vinegar and pepper. Serve the soup topped with the mushroom-walnut mixture and chives.
To make ahead: Prepare throuugh Step 4 and refrigerate for up to 2 days.