Preheat the oven to 375 degrees and lightly grease a 9x13 baking dish.

Bring a large pot of salted water to a boil and cook the spaghetti until just al dente. Drain and set aside.

While the pasta cooks, melt 1 tablespoon of butter in a skillet over medium heat. Add the sliced mushrooms and cook until they release their liquid and start to brown.

In a large mixing bowl, stir together the cream of mushroom soup, milk, garlic powder, salt, and pepper until smooth.

Add the cooked spaghetti, shredded turkey, frozen peas, cooked mushrooms, cheddar cheese, and half of the Parmesan to the bowl. Mix until everything is evenly coated.

Spread the mixture into the prepared baking dish.

In a small bowl, combine the breadcrumbs with the remaining tablespoon of melted butter and the rest of the Parmesan. Sprinkle this mixture evenly over the top.

Bake for 25 to 30 minutes until the casserole is hot and the topping is golden. Let it rest for a few minutes before serving.