Preheat your oven to 400°F (200°C).If you’re using raw chicken, slice the chicken breasts in half lengthwise. Pat them dry and season with salt, pepper, and Italian seasoning.
Heat olive oil in a pan over medium-high heat. Sear the chicken for 5-6 minutes on each side. Remove from the heat and set aside. Once cooled, dice it into bite-sized pieces.
In the same pot, lower the heat to medium-low and melt the butter. Use a spatula to clean any chicken bits from the bottom.
Add the diced onions, cooking until softened, about 4 minutes. Then, add minced garlic and cook for 1 more minute.
Sprinkle in the flour and mix it with the onions. Cook for 2 minutes, until you no longer smell flour.
Gradually add chicken broth while stirring. Keep the mixture thick and stir in milk a little at a time until smooth.
Let the sauce simmer while your pasta water heats up. Add the cream cheese and stir until fully melted.
Stir in shredded cheddar cheese. If you want a thinner sauce, add some juice from the diced tomatoes, or mix in the tomatoes and chicken.
Toss in the cooked Thin Spaghetti until well coated. If using a regular pot, transfer everything to a greased 9×13 casserole dish.
Top with shredded mozzarella cheese and bake uncovered for 15 minutes. Broil for 1-2 minutes for a golden top.
Once done, garnish with red pepper flakes and fresh parsley. Serve hot with garlic bread on the side.