Heat oven to 375ºF.
Bring a large pot of water seasoned with salt up to a rolling boil. Break the spaghetti in half and cook to al dente, save 1/2 cup of the cooking liquid in a separate bowl and drain the rest. After draining put the pasta back into the pasta pot.
In a large skillet over medium heat saute 1 tablespoon of butter and mushrooms until the mushrooms have browned, remove from pan, and set them aside.
Add four tablespoons of butter to the pan, once the butter has melted whisk in the flour and stir constantly for 2 minutes until the mixture is golden brown (this is called a roux).
Slowly whisk in the chicken stock, milk, heavy cream, garlic, salt, and pepper to the roux will constantly whisking to avoid lumps. Cook for 3-4 minutes until the mixture is bubbling and smooth.
Stir in the mushrooms, lemon juice, turkey, peas, carrots, and 3/4 cup of the parmesan cheese.
Pour the creamy turkey sauce into the pasta pot with the pasta and mix well. If the mixture is too thick add in some of the cooking water.
Pour the pasta and sauce mixture into a 9x13 greased pan.
In a small bowl combine the remaining cheese and bread crumbs.
Spread the mixture of the top of the pasta, bake for 30 minutes.