In heavy skillet over medium heat, saute mushrooms in butter until tender. Stir in salt/pepper.
Use portion of milk to dissolve cornstarch, then add remaining milk and cornstarch mixture, stirring constantly. Add Worcestershire sauce and bouillon and simmer until somewhat thickened. Add cheese, pepper and scallions to sauce.
Toss turkey and spaghetti in large saucepan, then add sauce, stirring well. Pour into greased 2-quart shallow casserole or baking dish. Sprinkle top with Parmesan and bake at 350F for about 20 minutes or until heated through.