Cook spaghetti; drain and set aside. Saute mushrooms in oil. Blend in flour, salt, and pepper; stir until smooth. Stir in broth slowly, cook until thickened. Stir 1 c. sauce into yogurt, then stir the yogurt into the sauce in pan. Add turkey, Parmesan cheese, and sauce to spaghetti. Put into a 9x13 inch pan, sprinkle top with Parmesan cheese. Bake at 425 degrees for 30 minutes.