Step 1: Sauté the Aromatics : In a large pot, heat butter and olive oil over medium heat. Add onions and cook until soft and translucent, about 5 minutes. Add garlic and cook for another minute until fragrant.

Step 2: Cook the Mushrooms : Add the sliced mushrooms to the pot and cook until they release their moisture and turn golden brown — about 10 minutes. Stir occasionally.

Step 3: Season and Thicken : Sprinkle in the flour (if using) and stir well for 1–2 minutes to create a light roux. Add thyme, salt, pepper, and soy sauce or Worcestershire sauce for an extra depth of flavor.

Step 4: Add the Broth and Simmer : Pour in the vegetable or chicken broth. Stir well, scraping up any browned bits from the bottom. Bring to a boil, then reduce the heat and let it simmer for about 15 minutes.

Step 5: Blend Until Smooth : Use an immersion blender to puree the soup until smooth (or carefully transfer it in batches to a blender).

Step 6: Add the Cream : Return the blended soup to the pot and stir in the heavy cream. Simmer gently for another 5 minutes. Taste and adjust salt and pepper as needed.