Preheat oven to 350˚ F . Lightly grease a 9-X-13-inch baking dish.

Add olive oil to a large saucepan set over medium-high heat. Add onions; saute until translucent. Add mushrooms; cook until golden. Turn heat to high; de-glaze pan with sherry. Scrape from pan; set aside. Add butter to pan over medium heat; melt. Add flour; stir until blended. Add chicken broth and milk. Cook and stir until mixture comes to a boil. Add reserved mushroom mixture and 1 1/2 cups Parmesan cheese; combine thoroughly, stirring until cheese is just melted. Remove from heat. Add turkey and spaghetti to mixture; combine thoroughly. Pour into baking dish.

Combine remaining Parmesan cheese and bread crumbs. Scatter mixture evenly over casserole. Bake until surface is light golden, about 20 minutes.