Preheat your oven to 350°F (175°C) and lightly grease a 9x13-inch baking dish with non-stick spray or butter.
Bring a large pot of salted water to a boil. Add the spaghetti and cook for 2 minutes less than the package directions for al dente; drain and set aside.
In a large skillet or Dutch oven, melt 4 tablespoons of butter over medium heat. Add the sliced mushrooms and diced onion, sautéing for 6 to 8 minutes until the mushrooms are browned and the onions are translucent.
Stir in the minced garlic and cook for 1 minute until fragrant. Sprinkle the flour over the vegetables and whisk constantly for 1 to 2 minutes to create a golden roux.
Slowly pour in the turkey broth and half-and-half while whisking continuously. Bring the mixture to a gentle simmer and cook for 3 to 5 minutes until the sauce has thickened enough to coat the back of a spoon.
Stir in the softened garlic-herb cream cheese, Italian seasoning, salt, and pepper until the cheese has melted and the sauce is smooth.
Add the cooked turkey, cooked spaghetti, and peas to the sauce. Toss well to ensure the pasta is thoroughly coated; the mixture should look slightly soupy as the pasta will absorb moisture during baking.
Transfer the mixture into the prepared baking dish. Top evenly with the shredded mozzarella and 1/4 cup of the parmesan cheese.
In a small bowl, toss the panko breadcrumbs with the remaining 1/4 cup of parmesan cheese and sprinkle the mixture evenly over the top of the casserole.
Bake for 25 to 30 minutes, or until the sauce is hot and bubbly and the breadcrumb topping is a deep golden brown. Let the dish rest for 5 minutes before serving.