Preheat the oven to 375°F and lightly grease a 9x9 baking dish.

Cook the spaghetti in a large pot of salted water until al dente, then drain and set aside.

Melt 1 tablespoon of butter in a skillet over medium heat. Add the onion and mushrooms and cook until softened.

Stir in the garlic and cook for one minute more.

Add the cream of mushroom soup, milk, salt, and pepper to the skillet. Stir until the sauce is smooth and heated through.

Remove the skillet from heat and mix in the cooked spaghetti, diced turkey, frozen peas, and Parmesan cheese until everything is evenly coated.

Transfer the mixture to the prepared baking dish and spread it into an even layer.

Melt the remaining tablespoon of butter and toss it with the breadcrumbs. Sprinkle the mixture over the top of the casserole.

Bake for 25 minutes until the topping is golden and the edges are bubbling.