Creamy Mushroom Soup

Heat 3 tablespoons oil in a large pot over medium heat. Add shallots and celery; cook, stirring occasionally, until tender, about 3 minutes. Add 4 cups each shiitakes and baby bellas, sherry and garlic; cook, stirring occasionally, until the mushrooms are soft and the liquid has evaporated, about 5 minutes. Stir in potatoes and thyme; cook for 1 minute. Add broth and water. Bring to a boil. Reduce heat to maintain a simmer and cook, stirring occasionally, until the vegetables are very soft, about 20 minutes.

Meanwhile, coarsely chop the remaining 2 cups mushrooms. Heat the remaining 1 tablespoon oil in a medium skillet over medium heat. Add the 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.