Melt 4 tablespoons of the butter in a heavy soup pot over medium heat.
Add the onions and thyme and cook (don’t brown) for 5 minutes, lowering the heat if needed.
Add the portabella mushrooms and cook for another 5 minutes
Add the porcini and their soaking water and raise the heat to a rapid simmer to reduce the liquid for 5 minutes.
Add the chicken stock and simmer for 1 hour
Remove the pot from the heat and puree the soup with an immersion blender.
Add the sherry, if using, and salt and pepper to taste.
In a separate saucepan, heat the remaining tablespoon of butter over medium-high heat and sear the additional 4 ounces of fresh mushrooms in the butter for a few minutes per side.
To serve, ladle out the soup and garnish with the parsley and a few of the seared mushrooms. Add a drizzle of truffle oil, if you wish!
Recipe by Sustainable Dish at https://sustainabledish.com/winter-mushroom-soup/