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 tsp salt, ½ tsp pepper, and the thyme. Cook until mushrooms are softened and starting to release thier 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 tsp of salt and bring to a strong simmer. Cook for 20 minutes, or until 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).