Melt the butter in a large pot or Dutch oven over medium heat.
Add the diced onion to the pot and sauté for about 5 minutes or until it becomes translucent.
Stir in the sliced mushrooms, cook for another 5-7 minutes, or until the mushrooms have released their moisture and start to brown. Add the minced garlic, sauté for one more minute.
Sprinkle the kosher salt over the mushroom mixture and stir well.
Sprinkle the all-purpose flour evenly over the mushroom mixture. Stir continuously for about 2 minutes to cook the flour and create a roux. It should thicken slightly.
Tie the sprigs of fresh thyme together with kitchen twine to create a bundle. Add the fresh thyme sprigs (or dried thyme) to the pot and pour in the dry sherry. Scrape the bottom of the pot with a wooden spoon to release any browned bits. Cook for 2-3 minutes until the sherry has reduced slightly.
Slowly pour in the chicken stock, stirring constantly to combine. Bring the mixture to a boil, then reduce the heat to low and let it simmer for 45 minutes to allow the flavors to meld together.
Remove the thyme sprigs if you used fresh thyme. Then, using an immersion blender or a regular blender (in batches if needed), carefully puree the soup until it's smooth and creamy.
Return the blended soup to the pot (if using a regular blender), and stir in the heavy whipping cream. Warm the soup over low heat, being careful not to bring it to a boil.
Season the bisque with salt and freshly ground black pepper to taste. Start with a pinch of each and adjust to your preference.
Once the bisque is heated through and seasoned to your liking, it's ready to serve. Ladle it into bowls and garnish with a few extra fresh thyme leaves if desired.