For the Soup: Melt the butter in a large stockpot. Add the garlic and shallots and cook until translucent. Add sliced morels and stir for 1 minute. Deglaze with sherry. Reduce until almost totally dry. Add the 2 quarts stock (rehydrated liquid) and bring to simmer, then reduce by 1/3. Slowly whisk in 2 cups of the heavy cream. Bring to boil. Add half of the minced thyme and simmer for 15 minutes. Whisk in 3, (1 teaspoon) balls of buerre manie to thicken. Simmer for 15 more minutes. Season with salt and freshly ground black pepper.
Thyme Cloud: Whip the remaining thyme with the 1/2 cup remaining whip cream until soft peak. Spoon on top of soup. Garnish with morel dust.
The mushroom stock is rehydrated morels. Soak dried morels in 2 quarts hot water for 15 minutes. Remove the mushrooms. This makes 7 ounces of rehydrated mushrooms and 2 quarts mushroom stock. (Chicken or vegetable stock can be substituted, however, much of the flavor for the soup is coming from the dried mushrooms which have been rehydrated.)