Preheat your oven to 350°F (175°C).
Cook the spaghetti according to package instructions until al dente; drain and set aside.
In a large mixing bowl, combine the shredded chicken, diced tomatoes, Italian seasoning, bell pepper, and onion.
Fold in the cooked spaghetti gently to combine.
In a separate bowl, mix the heavy cream, half of the mozzarella, and Parmesan cheese.
Add the cheese mixture to the spaghetti and chicken, stirring until well combined.
Transfer the mixture into a greased 9x13 inch casserole dish.
Top with the remaining mozzarella cheese.
Cover the dish with aluminum foil and bake for 30 minutes.
Remove the foil and bake for an additional 15-20 minutes or until the cheese is bubbly and golden.
Allow it to rest for 10 minutes before serving.