Halve chicken breasts lengthwise. Pat dry and lightly season with salt, pepper, and Italian seasoning.
Heat olive oil over medium-high heat
Sear the chicken on each side (5-6 minutes). Remove and set aside.
Reduce heat to medium-low. Melt the button and use a spatula to clean the chicken bits from the pan.
Add onions and cook until softened (about 4 minutes). Add garlic and cook for 1 minute.
Sprinkle flour over onions and toss to coat. Cook for 2 minutes.
Add chicken broth in splashes. Stir in between each splash to maintain the thickness of the roux.
Add milk (a little at a time). Whisk until smooth.
Start boiling water for pasta.
Cut chicken into bite-sized pieces.
Let the sauce simmer while pasta water heats up. Add the softened cream cheese and stir continuously until melted.
Reduce heat to low and let the base cool a bit. Sprinkle in shredded cheddar cheese and stir to combine.
Drain the Rotel and add to the sauce, along with the chicken. Stir to combine.
Preheat over to 400°
Cook spaghetti according to package directions.
Mix cooked spaghetti and sauce and transfer to a 9x13 casserole dish.
Top with mozzarella and bake uncovered for 15 minutes.
Remove from oven and garnish with red pepper flakes and/or fresh parsley.