Finely chop mushrooms, reserving 3-4 to be sliced and used later. Sauté chopped mushrooms, shallot and garlic in butter until softened, about 5 minutes. Sprinkle with flour and thyme; stir constantly, cooking 2-3 minutes until thick and bubbly. Whisk in broth, cream and sherry; bring to a gentle simmer for 2 minutes. Remove from heat and stir in sour cream. Season with freshly cracked black pepper.