Sauté aromatics: Heat the butter (3 tablespoons) and oil (1 tablespoon) in a large pot over medium heat. Add the onion (1) and cook for 5 minutes, stirring occasionally, until softened and lightly golden. Add the garlic (3 cloves) and cook 1 minute more.
Brown the mushrooms: Add half the mushrooms and cook undisturbed for 3–4 minutes to let them brown. Stir, then add the rest and cook another 5–6 minutes, until they’ve released their moisture and it’s mostly evaporated. Stir in soy sauce or Worcestershire (1 teaspoon).
Make the roux: Sprinkle the flour (1/4 cup) over the mushrooms and stir for 1 minute to coat and cook off the raw flour taste.
Deglaze: Pour in the white wine (1/3 cup), stirring constantly for 2–3 minutes, until the mixture thickens slightly and the wine has reduced by about half.
Simmer: Stir in both broths (2 cups chicken broth + 2 cups beef broth), Better Than Bouillon (2 teaspoons), thyme (1 teaspoon), and bay leaf (1 cup). Bring to a gentle simmer and cook 10–12 minutes, stirring occasionally, until slightly thickened and aromatic.
Blend for creaminess: Remove and discard the bay leaf. Transfer half the soup to a blender and puree until smooth, then return to the pot for a silky texture.
Finish with cream: Reduce the heat to low. Stir in the heavy cream (1 cup) and sherry (1 tablespoon). Warm gently for 1–2 minutes — do not let it boil. Taste and adjust seasoning with salt and pepper.
Serve: Ladle into bowls, garnish with parsley, and drizzle with olive oil or truffle oil for an aromatic finish.