Clean mushrooms by gently wiping with a damp paper towel. Slice most mushrooms thinly, but chop a quarter of them into small pieces for textural contrast.

In a large Dutch oven or heavy pot, melt butter over medium-high heat. Add mushrooms in a single layer. Don't stir immediately - let them brown for 3-4 minutes, then stir and continue cooking until golden and reduced in size, about 8-10 minutes total.

Reduce heat to medium and add diced onion to the mushrooms. Cook until onion is soft and translucent, about 4-5 minutes. Add minced garlic and cook for another 30 seconds until fragrant.

Sprinkle flour over the mushroom mixture and stir constantly for 1-2 minutes to cook out the raw flour taste.

Gradually pour in the broth while stirring continuously to prevent lumps. Add thyme and bay leaf. Bring to a simmer and reduce heat to low. Cover partially and cook for about 20 minutes to allow flavors to meld.

For a smoother consistency, remove bay leaf and purée about half of the soup using an immersion blender, or transfer in batches to a regular blender. Leave some chunks for texture.

Return soup to the pot (if removed). Stir in heavy cream and sherry (if using). Warm gently over low heat without boiling. Season with salt and pepper to taste.

Ladle hot soup into bowls and garnish with fresh parsley. A drizzle of extra cream or truffle oil makes an elegant finishing touch.