In a large Dutch oven or heavy-bottomed pot, melt 2 tablespoons of the butter over medium-high heat until it begins to brown slightly (creating brown butter). This step enhances the flavor. Add half of the chopped mushrooms. Cook, stirring occasionally, until they release their liquid and start to brown deeply (about 8-10 minutes). Remove the browned mushrooms with a slotted spoon and set aside; leave the flavorful browned butter in the pot.

Add the remaining 2 tablespoons of butter to the pot. Add the diced onion and sauté until softened and translucent, about 5 minutes. Add the minced garlic and the fresh thyme leaves, cooking for 1 minute until fragrant.

Add the remaining raw mushrooms to the pot and cook until they have softened and released their moisture, about 5-7 minutes. If using, pour in the sherry or wine to deglaze the pot, scraping up any browned bits from the bottom, and let it reduce by half.

Sprinkle the flour over the vegetables and cook, stirring constantly, for 1 minute to create a roux. Gradually whisk in the broth until smooth. Bring the mixture to a simmer, then reduce heat to low, cover partially, and cook for 15 minutes to allow flavors to meld.

Carefully transfer the soup mixture (including the cooked mushrooms and onions) to a blender, or use an immersion blender directly in the pot. Blend until mostly smooth, leaving a slight rustic texture. Return the soup to the pot if necessary.

Stir in the heavy cream, the reserved browned mushrooms, salt, and pepper. Heat gently until warmed through; do not allow the soup to boil after adding the cream. Taste and adjust seasoning as needed.

Ladle the soup into bowls. Garnish generously with chopped fresh chives and a small sprig of thyme before serving immediately.