In a large dutch oven, heat olive oil over medium high heat. Add onions and sauté until softened and slightly translucent. Add the mushrooms and salt, stir to combine. Cover and cook for 5-6 minutes until the mushrooms start to give off their liquid.
Meanwhile, secure a small piece of kitchen string around the thyme and rosemary. Set aside.
Add the chicken broth, bay leaf and thyme-rosemary bundle to the mushrooms. Stir to combine and simmer, covered for 15 minutes, until the herbs have wilted -- but haven't fallen off the stem. Remove the herbs and bay leaf and simmer uncovered for 5 more minutes. Remove from heat.
Working in batches, ladle the mushrooms and broth into a blender. Puree until smooth. Continue until all the soup has been pureed.
Add the cream and sherry and mix well. Taste for seasonings, adding salt and pepper if needed.
Reheat the soup and ladle into bowls, topping with a swirl of cream, chopped chives and additional sautéed mushrooms, if desired.