Sauté 1 cup of the chopped mushrooms separately in 1 tablespoon of butter until deeply browned and tender. Set these aside; they will be used as a chunky garnish later.

Melt the remaining 3 tablespoons of butter in a large Dutch oven or heavy-bottomed pot over medium heat. Add the diced onion and cook until translucent, about 5-7 minutes.

Add the remaining raw mushrooms and the minced garlic to the pot. Cook, stirring occasionally, until the mushrooms have released their liquid and started to brown, about 8-10 minutes. Stir in the fresh thyme.

Sprinkle the flour over the vegetables and stir constantly for 1 minute to cook out the raw flour taste (creating a roux). If using, deglaze the pot with Sherry or wine, scraping up any browned bits from the bottom.

Slowly whisk in the broth until completely smooth. Bring the soup to a gentle simmer, then reduce heat to low, cover, and cook for 15 minutes to allow flavors to meld.

Carefully transfer about two-thirds of the soup mixture to a blender (or use an immersion blender directly in the pot). Blend until perfectly smooth. Return blended mixture to the pot.

Stir in the heavy cream, Worcestershire sauce, salt, and pepper. Heat gently until warmed through, but do not allow to boil once the cream is added. Adjust seasoning as needed.

Ladle the hot soup into bowls. Top each serving generously with the reserved browned, chunky mushrooms.