In a large, heavy saucepan over medium heat, saute the mushrooms in the butter or margarine, stirring them often, until they are just tender.
Stir in the flour, salt, and pepper. Gradually add the milk, stirring constantly. Add the Worcestershire sauce, and simmer the sauce stirring it, until it has thickened somewhat.
Add the Swiss cheese or Cheddar, green pepper, scallions, and pimientos to the sauce, and mix the ingredients well. Stir in the turkey and spaghetti, combining the mixture well. Pour the mixture into a greased 2-quart shallow casserole or baking dish. Sprinkle the top of the casserole with the Parmesan.
Bake the casserole, uncovered, in a preheated 350°F oven for about 20 minutes or until it is heated through.