Preheat your oven to 350°F (175°C) and lightly grease a 9x13 baking dish with cooking spray or butter.

In a large pot, bring salted water to a boil, then add 12 ounces of spaghetti and cook according to package directions, reducing time by two minutes for al dente pasta. Drain but do not rinse.

In a skillet, heat 1 tablespoon of butter and 1 tablespoon of olive oil, sauté 1 finely diced onion and 1 diced bell pepper for 4-5 minutes. Stir in 3 minced garlic cloves and 1 teaspoon of Italian seasoning.

Lower the heat and stir in a 10.5-ounce can of cream of chicken soup, a 10-ounce can of diced tomatoes, ½ cup of chicken broth, ½ cup of sour cream, and 4 ounces of softened cream cheese until smooth.

Gently fold in 3 cups of cooked chicken and half of the 2 cups of shredded cheddar cheese into the sauce, then add the drained spaghetti and stir until coated.

Pour the mixture into the baking dish, spreading it evenly. Top with the remaining cheddar cheese.

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

Let the casserole rest for 10-15 minutes before optionally garnishing with 2 tablespoons of chopped fresh parsley.