In a large pot (i.e., Dutch oven), melt the butter over medium heat. Add the onions and cook until softened, about 4 to 5 minutes. Add the garlic and cook for 1 minute more.
Add the mushrooms, Worcestershire sauce, 1 teaspoon salt, ½ teaspoon pepper, and the thyme. Cook until mushrooms are softened and starting to release their liquid, about 8 to 10 minutes, stirring with a wooden spoon frequently.
If desired, use a slotted spoon to remove about 1 cup of the sauteed mushrooms. Set aside for garnish to the finished soup.
Add the flour to the pot and stir until the mushrooms are fully coated. Cook for about 1 minute.
Add the sherry and cook until reduced by about half, about 3 to 4 minutes.
Add the stock and another ½ teaspoon of salt and bring to a strong simmer. Cook for 20 minutes, or until the liquid has reduced somewhat and has thickened slightly.
Remove from heat and use a ladle to add half of the mushroom mixture to a blender or food processor, or bowl with an immersion blender. Puree the mixture until smooth.
Add the pureed mixture back into the pot with the reserved mushroom mixture. Turn the heat back on to medium.
Add the cream and nutmeg and stir until incorporated. Bring just to a simmer and then turn off the heat. Taste and add more salt, if desired.
Serve at once in individual soup bowls.
Garnish with reserved mushrooms, thyme, and grated nutmeg (if desired).
Video
Notes
Use a variety of mushrooms, if possible. We like a combination of button, shiitake, and baby Bella. Cut the stems from the shiitake before slicing, as they can sometimes be a little rubbery in the final dish.You can substitute a dry white wine or port for the sherry. If you prefer no alcohol at all, simply omit it. It adds a depth of flavor, but the soup will still be delicious without it. If using dried mushrooms, be sure to soak in warm water for at least 20 to 30 minutes before adding to the pot. Use the soaking liquid along with the stock when simmering the mushrooms.If you feel the soup is too thin, then just before adding the cream, take 1 tablespoon of cornstarch and 1 tablespoon of cold water and mix together, then pour into the soup. Stir until thickened. This will take a couple of minutes to thicken. If still not thick enough, repeat this process. Be careful not to add too much of the slurry, it will thicken up too much if you do!If you feel the soup is too thick, simply thin with a little more stock until the desired consistency is reached. The soup will keep in the refrigerator for up to 1 week and freezes nicely for up to 2 months.