Heat oven to 350. Cook spaghetti according to package directions. Rinse with hot water; drain. Set aside. Meanwhile, in 2 litre saucepan melt 2 tablespoons butter over medium heat; stir in flour until smooth and bubbly (1 minutes). With wire whisk, stir in chicken broth; continue cooking, stirring constantly, until mixture comes to a full boil (4 to 5 minutes). Boil 1 minutes. Remove from heat. Stir in 1.5 cups cheese and milk until cheese is melted. In greased 2 litre casserole combine cooked spaghetti, turkey, broccoli and mushrooms. Add cheese mixture. Top with remaining half cup cheese. Bake for 25 to 30 minutes or until bubbly around edge.