Preheat the oven to 400°F (200°C).

Cook the Chicken

Slice each chicken breast lengthwise into two thinner pieces. Season both sides lightly with salt, pepper, and Italian seasoning. Heat olive oil in a large pan over medium-high heat and sear the chicken for 5-6 minutes per side. Once done, remove from heat and set aside. Once cooled, dice into bite-sized pieces.

Prepare the Sauce

Lower the heat to medium-low, melt the butter in the same pan, and deglaze the bottom by scraping up any browned bits. Add the diced onions, cooking for 4 minutes until softened, then stir in the minced garlic and cook for another minute.

Sprinkle flour over the onions and stir to coat evenly, cooking for 2 minutes until the floury smell dissipates. Gradually add the chicken broth, stirring constantly between additions to maintain thickness. Next, slowly whisk in the milk until the mixture is smooth and well combined.

Cook the Spaghetti

As the sauce simmers and reduces, boil water in a large pot and cook the spaghetti according to package directions. Drain once done.

Finish the Sauce

Add the softened cream cheese to the sauce, stirring continuously until it’s fully melted and smooth. Lower the heat to a gentle simmer and allow the sauce to thicken. Gradually stir in the shredded cheddar cheese until fully incorporated.

Drain the diced tomatoes, reserving the juice if you prefer a thinner sauce, and stir them into the mixture along with the cooked chicken. Mix thoroughly.

Combine and Bake

Stir the cooked spaghetti into the sauce until evenly coated. If using a non-oven-safe pan, transfer everything to a greased 9x13-inch casserole dish.

Top with the shredded mozzarella cheese and bake, uncovered, for 15 minutes. To brown the cheese on top, broil for 1-2 minutes at 475°F, keeping a close eye to avoid burning.

Serve

Once out of the oven, garnish with a sprinkle of red pepper flakes and chopped parsley. Pair with garlic bread for a complete meal.