1. Preheat the oven to 400°F (200°C).

2. Slice each chicken breast lengthwise into two thinner pieces and season with salt, pepper, and Italian seasoning.

3. Heat olive oil in a large pan over medium-high heat and sear the chicken for 5–6 minutes per side. Set aside to cool, then dice into bite-sized pieces.

4. Lower the heat to medium-low. In the same pan, melt butter and deglaze by scraping up browned bits.

5. Add diced onions and cook for 4 minutes until softened. Stir in minced garlic and cook for 1 more minute.

6. Sprinkle flour over onions and stir to coat. Cook for 2 minutes to eliminate the raw flour taste.

7. Gradually add chicken broth, stirring between each addition. Slowly whisk in milk until smooth and combined.

8. While sauce simmers, cook spaghetti according to package directions, then drain.

9. Add softened cream cheese to the sauce, stirring until melted and smooth. Let the sauce thicken.

10. Stir in shredded cheddar cheese until incorporated.

11. Drain diced tomatoes and add them along with the diced chicken to the sauce. Mix well.

12. Combine the cooked spaghetti with the sauce until evenly coated.

13. If not using an oven-safe pan, transfer to a greased 9×13-inch casserole dish.

14. Top with shredded mozzarella and bake uncovered for 15 minutes.

15. Optional: Broil for 1–2 minutes at 475°F to brown the cheese, watching closely.

16. Garnish with red pepper flakes and chopped parsley. Serve hot.