Caramelize the onions: Melt 2 tbsp butter in a large, sturdy pot or Dutch oven over medium-low heat. Add the thinly sliced onions and a pinch of salt. Cook slowly, stirring every 5-10 minutes, for 30-40 minutes until deep brown and sweet. Add brown sugar substitute if using, and cook for 5 more minutes. Remove half of the caramelized onions and set aside for garnish; leave the other half in the pot.

Sauté the mushrooms: Increase heat to medium, add the remaining 4 tbsp of butter to the pot with the remaining onions. Add the chopped mushrooms and cook until they have released their moisture and started to brown, about 8-10 minutes. Stir in the minced garlic and fresh thyme, cooking for 1 minute until fragrant.

Create the roux and deglaze: Sprinkle the flour over the mushroom mixture and cook, stirring constantly, for 2 minutes to cook out the raw flour taste. If using sherry or wine, pour it in now and scrape up any browned bits from the bottom of the pot, allowing it to reduce for about 2 minutes.

Simmer the soup base: Gradually whisk in the broth until smooth. Bring the mixture to a gentle simmer, stirring occasionally, and let it cook for 15 minutes to allow the flavors to meld and the soup to thicken slightly.

Blend and finish: Remove the pot from the heat. Stir in the heavy cream and Worcestershire sauce. Use an immersion blender to blend the soup until it reaches your desired consistency (partially smooth for texture, or fully smooth). Alternatively, carefully transfer batches to a standard blender.

Season and serve: Heat the soup gently until warmed through (do not boil after adding cream). Season generously with salt and pepper to taste. Ladle into bowls and garnish each serving with the reserved caramelized onions and an optional drizzle of truffle oil.