Prep the chicken
Pat the chicken breasts dry, then season both sides with salt, pepper and the Italian seasoning. Slice each breast in half horizontally if they’re thick so they cook more evenly.

Sear the chicken
Heat the olive oil in a large skillet over medium-high heat. Add the chicken and cook until nicely browned and done through, about 5–6 minutes per side depending on thickness. Remove the chicken to a plate to rest, then cut into bite-sized pieces.

Start the sauce base
Lower the heat to medium. In the same skillet, melt the butter. Add the chopped onion and cook until softened, about 4 minutes. Stir in the minced garlic and cook 30–60 seconds until fragrant.

Thicken the sauce
Sprinkle the flour over the onions and garlic and stir to coat. Cook, stirring, for about 1–2 minutes to remove the raw flour taste. Slowly whisk in the chicken broth until smooth, then whisk in the milk. Bring the mixture to a gentle simmer so it thickens slightly.

Add the cream cheese and cheddar
Reduce heat to low and stir the softened cream cheese into the sauce until it melts and the mixture is smooth. Add the shredded cheddar a handful at a time, stirring until fully incorporated and velvety.

Cook the pasta
While the sauce comes together, boil the spaghetti in a large pot of salted water according to package directions until al dente. Drain and return to the pot.

Combine tomatoes and chicken with the sauce
Stir the drained diced tomatoes and the chopped cooked chicken into the cheese sauce. If you want a thinner sauce, you can stir in a little of the reserved tomato juice or extra broth.

Toss pasta with sauce
Pour the sauce over the drained spaghetti and toss gently until all the noodles are coated.

Bake to finish
If your skillet is ovenproof, leave the mixture in it; otherwise transfer everything to a greased 9×13-inch baking dish. Sprinkle the shredded mozzarella evenly over the top. Bake in a preheated 400°F (200°C) oven for about 15 minutes, until heated through and the cheese is bubbly.

Brown the top (optional)
For a golden finish, switch the oven to broil and place the casserole under the broiler for 1–2 minutes — watch it closely so it doesn’t burn.

Serve
Remove from the oven, let rest a couple minutes, then garnish with chopped parsley and a pinch of red pepper flakes if desired. Serve with garlic bread or a crisp salad.