Preheat your oven to 400°F (200°C).
Cook the Chicken: If using uncooked chicken breasts, slice them in half lengthwise to create two thinner slices. Pat them dry and season lightly with salt, pepper, and Italian seasoning.
In a large skillet, heat olive oil over medium-high heat. Sear the chicken on each side for 5-6 minutes until golden brown. Remove the chicken from the skillet and set it aside to cool. Once cooled, dice the chicken into bite-sized pieces.
Make the Sauce: Reduce the heat to medium-low. Melt the butter in the same skillet, scraping the bottom to incorporate the flavorful chicken bits.
Add the diced onions and cook until softened, approximately 4 minutes. Stir in the minced garlic and cook for an additional minute.
Sprinkle the flour over the onions and garlic, tossing to coat. Cook for 2 minutes until the raw flour smell disappears.
Gradually add the chicken broth to the skillet, stirring between each addition to maintain the thickness of the roux. Next, add the milk slowly, whisking continuously until the mixture is smooth and well combined.
While the sauce simmers, cook the spaghetti according to the package instructions until al dente. Drain the cooked pasta and set it aside.
Add the softened cream cheese to the skillet, stirring continuously until it melts and the sauce becomes creamy. If the cream cheese is cold, it may take a bit longer to melt.
Reduce the heat to low and let the sauce cool slightly. Sprinkle in the shredded cheddar cheese and stir until it melts and incorporates into the sauce.
If you prefer a thinner sauce, you can add a little bit of the juice from the diced tomatoes. Otherwise, drain the juice and add the tomatoes along with the diced chicken to the sauce. Stir to combine.
Stir in the cooked spaghetti. If your skillet is not oven-safe, transfer the mixture to a lightly greased 9x13-inch casserole dish.
Bake: Top the dish with shredded mozzarella cheese and bake, uncovered, for 15 minutes until the cheese is melted and bubbly. For an extra golden touch, you can broil the dish for a few minutes at 475 degrees Fahrenheit (245 degrees Celsius), keeping a close eye on it.
Remove the dish from the oven and garnish with red pepper flakes and freshly chopped parsley. Serve alongside garlic bread with cheese for a complete and satisfying meal.