Step 1: Cook the Pasta

1. Bring a large pot of salted water to a boil and cook the spaghetti until al dente according to the package instructions.
2. Drain the pasta and set aside. Drizzle with a little olive oil to prevent sticking.

Step 2: Sauté the Vegetables

1. In a large skillet or sauté pan, heat the butter and olive oil over medium heat.
2. Add the diced onion and cook for 3-4 minutes, until softened.
3. Stir in the minced garlic and sliced mushrooms. Cook for another 5-7 minutes, or until the mushrooms are golden and tender.

Step 3: Make the Creamy Sauce

1. Sprinkle the flour over the cooked vegetables and stir to coat. Cook for 1-2 minutes to remove the raw flour taste.
2. Gradually whisk in the chicken or turkey broth, stirring constantly to prevent lumps. Bring to a simmer.
3. Reduce the heat to low and stir in the heavy cream, sour cream, and Dijon mustard. Let the sauce simmer for 2-3 minutes until thickened.

Step 4: Add the Turkey and Pasta

1. Stir the shredded turkey, frozen peas (if using), Parmesan cheese, and dried thyme into the sauce. Season with salt, pepper, and smoked paprika (if using).
2. Add the cooked spaghetti to the skillet and toss to coat evenly with the sauce.

Step 5: Assemble the Casserole

1. Preheat your oven to 375°F (190°C).
2. Transfer the turkey and pasta mixture to a greased 9×13-inch baking dish.
3. Sprinkle the mozzarella cheese evenly over the top.

Step 6: Prepare the Topping

1. In a small bowl, mix the panko breadcrumbs with the melted butter and Parmesan cheese.
2. Sprinkle the breadcrumb mixture evenly over the casserole.

Step 7: Bake

1. Place the casserole in the preheated oven and bake for 20-25 minutes, or until the top is golden brown and bubbly.
2. For an extra crispy topping, broil for 1-2 minutes at the end, watching carefully to prevent burning.

Step 8: Serve

1. Remove the casserole from the oven and let it cool for 5 minutes before serving.
2. Garnish with chopped parsley, if desired, and serve warm with a side salad or garlic bread.