Bring a large pot of salted water to a boil and add the spaghetti. Cook until al dente, following package instructions but subtracting 2 minutes. Drain the pasta without rinsing and let it cool slightly.

Dice the onion and bell pepper into small pieces, and mince the garlic finely. Shred the cooked chicken into bite-sized pieces.

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

In a large skillet, heat butter and olive oil over medium heat. Add the diced onion and bell pepper, cooking until they soften. Stir in the minced garlic and Italian seasoning, cooking for an additional minute.

Stir in the cream of chicken soup and the can of Rotel. Pour in the chicken broth and mix until smooth. Add in the cream cheese, stirring until fully combined.

Fold in the shredded chicken and cooked spaghetti into the sauce mixture until everything is thoroughly coated.

Pour the mixture into the greased baking dish, spreading it evenly. Sprinkle the remaining cheddar cheese over the top.

Bake uncovered for 25-30 minutes until edges bubble and cheese is golden brown. Let it rest for 10-15 minutes before serving.