01 -
Preheat oven to 400 degrees Fahrenheit.
02 -
Slice the chicken breasts in half lengthwise to create thinner slices. Pat dry and season with salt, pepper, and Italian seasoning. Heat olive oil in a pan over medium-high heat. Sear chicken for 5-6 minutes on each side. Remove from the pan, let cool, and dice into bite-sized pieces.
03 -
Reduce heat to medium-low. Melt butter in the same pan, scraping up any browned bits from the chicken. Add onions and cook until softened, about 4 minutes. Stir in garlic and cook for an additional minute.
04 -
Sprinkle flour over the onions and stir to coat. Cook for 2 minutes until the raw flour smell subsides.
05 -
Gradually add chicken broth in small splashes, stirring between each addition to maintain a smooth consistency. Add milk in the same manner, whisking continuously until smooth.
06 -
While the sauce simmers, bring a pot of water to a boil. Cook spaghetti according to package instructions, then drain.
07 -
Add softened cream cheese to the sauce and stir until fully melted and smooth. Sprinkle in shredded cheddar cheese and stir to combine. Add the diced tomatoes along with the cooked chicken. Stir to combine.
08 -
Add cooked spaghetti to the sauce and mix well. If the pan is not oven-safe, transfer the mixture to a greased 9x13-inch casserole dish.
09 -
Top with mozzarella cheese. Bake uncovered for 15 minutes. For a browned top, broil at 475 degrees Fahrenheit for 1-2 minutes, keeping a close watch.
10 -
Remove from the oven and garnish with red pepper flakes and chopped parsley. Serve with garlic bread if desired.