Preheat oven to 350°F (175°C).

Grease a 9x13-inch baking dish.

Cook the Pasta

Cook spaghetti 2 minutes less than the package directions (it will finish cooking in the oven).

Drain and set aside.

Sauté the Vegetables

In a large skillet, heat olive oil over medium heat.

Add diced onion and bell pepper, cooking for about 3-4 minutes until softened.

Stir in garlic and cook for another 30 seconds. Remove from heat.

Make the Sauce

In a large mixing bowl, whisk together cream of chicken soup, cream of mushroom soup, sour cream, and chicken broth until smooth.

Add 1 cup shredded cheddar, mozzarella, salt, pepper, smoked paprika, garlic powder, onion powder, Italian seasoning, and red pepper flakes (if using). Mix well.

Assemble the Casserole

Stir the cooked spaghetti, shredded chicken, and sautéed vegetables into the sauce. Mix until well coated.

Transfer the mixture to the prepared 9x13-inch baking dish.

Sprinkle the top with the remaining ½ cup cheddar cheese and Parmesan cheese.

Optional Crispy Topping

If using breadcrumbs or crushed crackers, toss them with melted butter and sprinkle evenly over the cheese layer.

Bake

Bake uncovered for 25-30 minutes until bubbly and golden.