Melt the butter in a large Dutch oven or heavy-bottomed pot over medium heat. Add the chopped onion and sauté for 5-7 minutes until softened and translucent.

Add the minced garlic and chopped mushrooms to the pot. Cook, stirring occasionally, for about 10-12 minutes, until the mushrooms have released their liquid and started to brown nicely. Stir in the dried thyme.

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

Slowly whisk in the broth, ensuring no lumps form. Bring the mixture to a gentle simmer, scraping up any browned bits from the bottom of the pot. Let simmer for 10 minutes to allow the soup to thicken slightly and the flavors to meld.

Remove the pot from the heat. Stir in the heavy cream and Worcestershire sauce (if using). Season generously with salt and pepper to taste.

Add the fresh spinach in batches, stirring until it has completely wilted into the hot soup. This typically takes 1-2 minutes.

For a smoother consistency, use an immersion blender to partially blend the soup until desired texture is reached (leave some chunks for texture). Serve hot, garnished with fresh parsley or croutons if desired.