Place the dried porcini mushrooms in a small bowl and cover with 1 cup of hot water. Let them soak for 20 minutes. Remove the softened mushrooms with a slotted spoon, chop them finely, and set aside. Carefully pour the soaking liquid into another container, leaving any grit behind at the bottom of the bowl.

In a large pot or Dutch oven, melt the butter with the olive oil over medium heat. Add the chopped shallots and cook until softened, about 3-4 minutes. Stir in the minced garlic and cook for another minute until fragrant.

Increase the heat to medium-high. Add the sliced cremini and shiitake mushrooms, salt, and pepper. Cook, stirring occasionally, until the mushrooms have released their liquid and have started to brown and caramelize, about 8-10 minutes. This browning step is key for a deep flavor.

Stir in the chopped rehydrated porcini mushrooms. Sprinkle the all-purpose flour over the mushroom mixture and cook, stirring constantly, for 1 minute to cook out the raw flour taste.

If using, pour in the dry sherry to deglaze the pot, scraping up any browned bits from the bottom. Let it bubble and reduce for about a minute. Slowly whisk in the vegetable broth and the reserved porcini soaking liquid. Add the fresh thyme sprigs.

Bring the soup to a boil, then reduce the heat to low, cover, and let it simmer for 15 minutes to allow the flavors to meld together.

Remove the pot from the heat and discard the thyme sprigs. Use an immersion blender to blend the soup directly in the pot until it reaches your desired consistency. For a very smooth soup, you can carefully transfer it to a standard blender in batches.

Return the pot to low heat. Stir in the heavy cream and heat through gently, but do not let it come to a boil. Taste and adjust seasoning with more salt and pepper if needed.

Ladle the soup into bowls and garnish with fresh chopped chives or parsley before serving.