Cook spaghetti al dente, 8-10 minutes, or according to package directions. Drain pasta and do not rinse.
In a large skillet or pot melt butter over medium heat. Add onion, garlic, and mushrooms and cook for about 5 minutes, until onion is soft.
Add flour and cook for one minute while stirring.
Add broth and cream and whisk (or stir) until thickened and bubbling. Boil for 1 minute then stir in cream cheese until melted. Add parsley, Italian dressing mix, salt, and pepper.
Combine sauce with cooked spaghetti, turkey, 1 cup mozzarella, and ¼ cup Parmesan. Add spaghetti mixture to baking pan and top with remaining mozzarella cheese.
Cover and bake 20 minutes. Sprinkle with remaining Parmesan and bake uncovered for an additional 10-15 minutes, or until bubbling.
Allow to rest 5 minutes before serving.
Notes
Turkey tetrazzini can be prepped ahead and stored in the refridgerator for up to 2 days. Before baking, just top with the remaining cheese.
Refrigerate leftovers for up to 3 days covered with plastic wrap, or in a covered container. Freeze for up to 1 month, and thaw in the refrigerator.
Reheat leftovers in the microwave or on the stovetop. Add a little cream to loosen up the cheesy sauce.