Bring a large pot of water to a boil. Add 1/2 TBS salt & uncooked spaghetti. Cook until spaghetti is al dente. Drain & leave in pot. Set aside.
In a large deep skillet add butter and allow to sizzle. Add in bell pepper and onion and cook until softened. Season with 1/8 tsp salt & 1/4 tsp pepper. Stir in minced garlic for 30 seconds. Add in all-purpose flour & stir for 1 minute.
Whisk in heavy whipping cream, chicken broth, and milk until thickened. Season with Italian seasoning. Taste for seasoning adjustments if needed. Turn off heat & remove from burner.
Add in canned diced tomatoes with juice, 4oz Italian blend shredded cheese, shredded cooked chicken, and cooked & drained spaghetti. Stir until well blended.
Pour mixture into a prepared (spray with non-stick cooking spray) 9 x 13 baking dish evenly. Top with remaining 4 oz Italian blend cheese.
Bake in 375 degree preheated oven for 15-20 minutes until cheese has melted and casserole is heated through & bubbly. Garnish with fresh parsley if desired. Allow to rest for 5 minutes and cut into squares. Enjoy.