In a large Dutch oven or heavy-bottomed pot, melt the butter over medium heat. Add the diced onion and cook until softened and translucent, about 5-7 minutes.

Add the chopped mushrooms and dried thyme to the pot. Sauté for 10-12 minutes, stirring occasionally, until the mushrooms have released their liquid and started to brown and caramelize slightly. Add the minced garlic during the last minute of cooking.

Sprinkle the flour over the mushroom mixture and stir constantly for 1 minute to cook out the raw flour taste, creating a roux base.

Gradually whisk in the broth, ensuring no lumps form. Bring the mixture to a simmer, then reduce the heat to low, cover, and let it cook gently for 15 minutes to allow the flavors to meld.

Carefully transfer the soup mixture to a blender (working in batches if necessary) or use an immersion blender directly in the pot. Blend until relatively smooth; for a chunkier texture, blend only partially.

Return the pureed soup to the pot over low heat. Stir in the heavy cream and Worcestershire sauce. Heat through gently without boiling. Season with salt and pepper to taste.

Ladle the hot soup into bowls. Serve immediately with a dollop or generous swirl of cold sour cream on top of each serving.