Cook the mushrooms in butter in a 2 - 3 quart casserole for 5 minutes. Add flour and stir until smooth. Add stock and cream and cook, stirring constantly, until thickened. Beat yolks and add a little of the sauce to them, mixing well. Return to sauce and stir to blend. Do not allow to boil. Add turkey and spaghetti and mix well. Sprinkle with cheese and bake at 400ºF. oven until golden brown, about 20 - 25 minutes. Serves 6