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

Add the sliced mushrooms and dried thyme to the pot. Increase the heat slightly to medium-high. Cook, stirring occasionally, until the mushrooms have released their liquid and started to brown richly (about 10-12 minutes). Stir in the minced garlic for the last minute.

Sprinkle the flour over the mushroom mixture (creating a roux) and stir constantly for 1-2 minutes to cook out the raw flour taste. Slowly whisk in the broth, ensuring no lumps form. Bring the soup to a simmer.

Reduce heat to low, cover partially, and simmer for 15 minutes, allowing the flavors to meld. Season generously with salt, pepper, and Worcestershire sauce (if using).

Remove the pot from the heat. Use an immersion blender to blend about half the soup until smooth, leaving the rest chunky for texture. Alternatively, carefully transfer half the soup to a standard blender, blend until smooth, and return it to the pot.

Stir in the heavy cream. Heat gently until just warmed through (do not boil after adding the cream). Taste and adjust seasoning as needed. Let cool completely before portioning into airtight containers for meal prepping.