Sprinkle the mushrooms with lemon juice. In a food processor, coarsely chop them.
Melt the butter in a heavy saucepan and lightly saute the shallots. Add the mushrooms, thyme, and bay leaf and saute over moderate heat for 10 minutes, or until the liquid disappears. Add the salt, pepper, cream, and chicken stock and bring to a boil. Reduce the heat and simmer for 20 minutes.
Add the cornstarch to the soup and continue to simmer 10 minutes longer, stirring constantly. Correct seasoning to taste.
Serve in warm bowls. Sprinkle with minced parsley.
Recipe courtesy of Wolfgang Puck, Modern French Cooking for the American Kitchen: Houghton Mifflin, 1981