Purée of Portabello Mushroom Soup

Ingredients

• extra virgin olive oil
  2 tablespoons
• butter (optional)
  1 tablespoon
• diced onions
  1 cup
• diced celery
  ½ cup
• diced leeks
  ½ cup
• portabello mushrooms

  gills removed, trimmed and diced
  1 lb
• chopped garlic

  to taste
  ½ teaspoon
• chopped fresh thyme leaves
  ¼ teaspoon
• dry sherry
  ½ cup
• vegetable stock
  6 cups
• salt and freshly ground black pepper
  1 unit
• freshly grated Parmesan cheese
  ½ cup