Prepare the cashew cream: Place soaked and drained cashews in a high-speed blender with 1/2 cup of fresh water. Blend until completely smooth and creamy, adding a tablespoon of water at a time if necessary to reach a thick, smooth consistency. Set aside.

Sauté aromatics: In a large pot or Dutch oven, heat the olive oil over medium heat. Add the chopped onion and cook until softened and translucent, about 5-7 minutes. Add the minced garlic and cook for 1 minute more until fragrant.

Cook mushrooms: Add the sliced mushrooms and dried thyme to the pot. Cook, stirring occasionally, until the mushrooms have released their moisture and started to brown, about 10-12 minutes.

Create the roux and deglaze: Sprinkle the flour over the mushrooms and stir constantly for 1 minute. If using, carefully pour in the dry sherry, scraping up any browned bits from the bottom of the pot, and let it cook down for 1 minute.

Simmer the soup: Gradually whisk in the vegetable broth until smooth. Bring the mixture to a boil, then reduce heat and simmer for 10 minutes to allow the flavors to meld. Stir in the nutritional yeast.

Blend and finish: Carefully transfer about half of the soup mixture to a blender (or use an immersion blender directly in the pot). Add half of the reserved cashew cream and blend until smooth. Return the blended portion to the pot (if using a standing blender). Stir well and add more cashew cream as desired for increased richness. Season generously with salt and black pepper to taste.