Cook the spaghetti according to package directions. Drain and set aside.
2. Make the sauce:
In a large mixing bowl, combine the cream of mushroom soup, sour cream, and chicken broth. Stir in the garlic powder, black pepper, and paprika (if using).
Add the shredded chicken, frozen peas (if using), and chopped onions (if using) to the mixture. Stir to combine.
3. Combine with pasta:
Add the cooked spaghetti to the sauce mixture and toss gently to coat the pasta and chicken evenly with the sauce.
4. Assemble the casserole:
Grease a 9×13-inch baking dish with butter or cooking spray.
Transfer the pasta and chicken mixture into the prepared baking dish.
Sprinkle shredded mozzarella cheese and Parmesan cheese evenly over the top. If you like a crunchy topping, you can sprinkle breadcrumbs over the cheese.
5. Bake:
Cover the baking dish with aluminum foil and bake in the preheated oven for 20-25 minutes.
Remove the foil and bake for an additional 10-15 minutes, or until the cheese is melted and bubbly and the top is golden brown.
6. Serve:
Let the casserole cool for a few minutes before serving. Garnish with fresh parsley if desired.
