Prepare the produce: clean and thinly slice the Mushrooms, finely chop the Yellow Onion, and mince the Garlic.
Melt the Butter in a large heavy-bottomed pot over medium heat.
Add the chopped onion and cook until softened and translucent, about 5 minutes, then add the minced garlic and cook another 30–60 seconds until fragrant.
Add the sliced mushrooms and increase the heat to medium-high; sauté until they release their moisture and begin to brown, about 8–10 minutes. If desired, reserve a few browned mushroom slices for garnish.
Sprinkle the All-Purpose Flour over the mushroom mixture and cook, stirring, for 1–2 minutes to remove the raw flour taste and create a light roux.
Slowly whisk in the Chicken Broth so there are no lumps, then add the Dried Thyme and the Bay Leaf. Bring the soup to a gentle simmer and cook for 10–15 minutes to let the flavors meld and the soup thicken slightly.
Stir in the Dry Sherry and season with Kosher Salt and freshly ground Black Pepper to taste. Adjust seasoning after the next step if needed.
Remove the bay leaf and, using an immersion blender, blend the soup until smooth or to your desired texture. Alternatively, carefully blend in batches in a countertop blender and return to the pot.
Stir in the Heavy Cream and warm gently — do not boil — until the soup is heated through and silky. Taste and adjust salt and pepper.
Ladle into bowls, garnish with the reserved sautéed mushrooms and an extra grind of black pepper if desired, and serve hot.
