Melt the butter in a heavy stockpot over medium-high heat. When it froths, turn down the heat to medium-low, stir in the shallot and thyme
, and sauté until fragrant, about 3 minutes. Toss the mushrooms into the pot and sprinkle with the salt and white pepper. Cover the pot and sweat the mushrooms until tender, about 10 minutes. Stir in the broth and simmer, uncovered, for 20 minutes. Turn off the heat and puree the soup with an immersion blender until smooth. Stir in the sherry and heavy cream and serve.