Preheat oven to 400°F (200°C). Toss the roughly chopped mushrooms (reserving about 1 cup for topping later) with 2 Tbsp olive oil, 1 tsp salt, and 0.5 tsp pepper on a large baking sheet.

Roast the mushrooms for 25-30 minutes, stirring halfway through, until deeply browned and caramelized. Remove the reserved 1 cup of mushrooms and set aside. These will be the final topping.

While the main batch roasts, melt the butter in a large Dutch oven or pot over medium heat. Add the diced onion and sauté until softened and translucent, about 6-8 minutes.

Add the minced garlic and thyme; cook for 1 minute until fragrant. Stir in the flour and cook for 2 minutes, creating a roux. If using, pour in the sherry/wine and scrape up any browned bits from the bottom of the pot, cooking until the liquid has almost evaporated.

Gradually whisk in the broth until smooth. Bring the mixture to a simmer, then add the remaining majority roasted mushrooms (leaving the topping portion aside). Simmer gently, stirring occasionally, for 15 minutes to allow flavors to meld.

Remove the soup from the heat. Carefully use an immersion blender to blend until smooth, or transfer in batches to a regular blender. Return the puréed soup to the pot over low heat.

Stir in the heavy cream. Taste and adjust seasoning with salt and pepper as needed. Heat through gently but do not boil after adding the cream.

While the soup finishes heating, quickly sauté the reserved 1 cup of mushrooms in a small, dry skillet over medium-high heat for 3-4 minutes until extra crispy. This adds texture to the garnish.

Ladle the soup into bowls. Top each serving generously with the crispy roasted mushrooms and a sprinkle of fresh parsley before serving hot.