Step 1: Cook the Spaghetti

1. Bring a large pot of salted water to a boil. Add the spaghetti and cook according to the package instructions until just shy of al dente (the pasta will finish cooking in the oven).
2. Drain the spaghetti and toss with a drizzle of olive oil to prevent sticking. Set aside.

Step 2: Prepare the Vegetables

1. Heat olive oil or butter in a large skillet over medium heat.
2. Add the chopped onion and bell pepper, cooking for 4-5 minutes until softened.
3. Stir in the minced garlic and cook for an additional 1 minute, until fragrant. Remove from heat and set aside.

Step 3: Make the Creamy Sauce

1. In a large mixing bowl, combine the cream of chicken soup, cream of mushroom soup, chicken broth, sour cream, paprika, onion powder, garlic powder, black pepper, and salt. Whisk until smooth and well combined.
2. Add the cooked vegetables to the sauce mixture and stir to combine.

Step 4: Assemble the Casserole

1. Preheat your oven to 375°F (190°C). Grease a 9×13-inch baking dish with nonstick spray or butter.
2. In a large mixing bowl, combine the cooked spaghetti, shredded chicken, and half of the shredded cheddar cheese. Pour the creamy sauce over the mixture and toss until everything is evenly coated.
3. Transfer the mixture to the prepared baking dish, spreading it out into an even layer.

Step 5: Add the Cheese and Topping

1. Sprinkle the remaining cheddar cheese and mozzarella cheese evenly over the casserole.
2. If using the breadcrumb topping, mix the breadcrumbs (or crushed crackers) with melted butter and Parmesan cheese. Sprinkle this mixture over the top of the cheese layer.

Step 6: Bake the Casserole

1. Place the casserole in the preheated oven and bake uncovered for 30-35 minutes, or until the cheese is melted, bubbly, and golden brown.
2. If desired, broil the casserole for 1-2 minutes at the end of baking to achieve a crispy, golden crust.

Step 7: Garnish and Serve

1. Remove the casserole from the oven and let it rest for 5-10 minutes before serving.
2. Garnish with freshly chopped parsley or red pepper flakes, if desired. Serve warm and enjoy!