Step 1: Cook the Spaghetti

1. Bring a large pot of salted water to a boil.

2. Add spaghetti and cook until al dente according to package directions.

3. Drain and set aside.

Step 2: Sauté the Veggies

1. While the pasta cooks, heat olive oil or butter in a skillet over medium heat.

2. Add chopped onion and bell pepper; sauté for 4–5 minutes until soft.

3. Stir in garlic and cook for another 30 seconds until fragrant.

4. Remove from heat and let cool slightly.

Step 3: Mix the Casserole Filling

1. In a large bowl, whisk together cream of mushroom soup, cream of chicken soup, sour cream, chicken broth (or milk), Italian seasoning, salt, and pepper.

2. Stir in the sautéed vegetables and shredded chicken.

3. Add 1½ cups cheddar cheese, mozzarella, and cooked spaghetti. Mix everything until evenly combined.

Step 4: Assemble the Casserole

1. Preheat oven to 350°F (175°C).

2. Lightly grease a 9×13-inch baking dish.

3. Pour the chicken spaghetti mixture into the dish and spread evenly.

4. Top with remaining ½ cup cheddar cheese and Parmesan (if using).

Step 5: Bake and Serve

1. Bake uncovered for 25–30 minutes, or until bubbly and golden around the edges.

2. Let cool for 5–10 minutes before serving.

3. Garnish with chopped parsley or a sprinkle of red pepper flakes if desired.