Preheat oven to 350°F (175°C).

Cook spaghetti according to package directions.

Drain the cooked spaghetti.

Grease an 11-inch baking dish.

Place the drained spaghetti in the greased baking dish.

Top the spaghetti with cubed cooked turkey.

Set aside the baking dish with spaghetti and turkey.

In a skillet, melt butter over medium heat.

Add sliced mushrooms and chopped onion to the skillet.

Sauté the mushrooms and onion in butter until tender.

In a separate bowl, whisk together cream of mushroom soup, milk, poultry seasoning, and ground mustard until blended.

Pour the soup mixture into the skillet with the sautéed mushrooms and onions.

Stir in the cheddar cheese and cook over medium heat until the cheese is melted.

Pour the cheese sauce over the turkey in the baking dish.

Sprinkle the remaining shredded mozzarella and parmesan cheese over the top of the casserole.

Bake, uncovered, at 350°F (175°C) for 25-30 minutes.

Bake until the casserole is heated through and the cheese is melted and bubbly.

Let cool slightly before serving.