Butterfly the chicken breasts to create 4 thinner breasts. Sprinkle Italian seasoning, salt and pepper evenly on both sides of the breasts.
Meanwhile, using a large, deep skillet or Dutch oven, add olive oil and place over medium-high heat. Once hot, add the chicken and cook for 10-12 minutes, flipping halfway through, or until chicken is fully cooked. Once cooked, remove chicken and set aside to cool.
Bring a large pot of salted water to a boil. (Note: This will be used to cook the spaghetti later.)
Using the same pan, add butter, onions and bell pepper; sauté for 4-5 minutes, or until onions have softened.
Add garlic, stir and continue sauteing for 1-2 more minutes.
Sprinkle flour evenly on top of onions and garlic; stir until well combined. Continue sauteing for 1-2 more minutes.
Add chicken broth and milk; stir until well combined. Cook, stirring occasionally, for 4-5 minutes.
Add cream cheese; stir until completely melted. Reduce heat to low.
Let sauce continue cooking on low heat; Cook spaghetti until al dente according to package instructions. Once cooked, drain and set aside.
Add cheddar cheese to sauce; stir until completely melted.
Add diced tomatoes and chilies; stir until well combined.
Add cooked spaghetti; stir until well combined.
Chop chicken into bite-sized pieces and add to the pan; stir until well combined. (Note: If using a Dutch oven, you can place it directly into the oven. If using a large skillet, transfer mixture into a greased 9”x13” casserole dish.)
Sprinkle mozzarella cheese evenly on top.
Bake uncovered for 15-17 minutes.
Remove from oven and top with chopped parsley and red pepper flakes before serving.