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.
For the Swiss sherry chicken...
Preheat oven to 350 degrees. Coat baking dish with cooking spray.
Pound chicken breasts between two pieces of plastic wrap to an even thickness (about 1-inch). Depending on size, you can cut each breast in half. Place chicken in baking dish; top each piece with Swiss cheese. Pour mushroom sauce over chicken and cheese. Toss toasted croutons with melted butter and sprinkle over top.
Bake for 40 minutes or until chicken is cooked through. Allow to rest for 5 minutes before serving.
While chicken is baking, sauté reserved sliced mushrooms until golden brown. Serve chicken sprinkled with sautéed mushroom and chopped parsley.