Chop up the onion, carrots and celery into bite sized pieces.
Add the veggies to a large casserole dish that you have sprayed with cooking spray.
Add the rice and broth to the baking dish with the veggies. Cover the baking dish tightly with foil.
Bake at 350 degrees for 1 hour and 15 minutes.
Check on the rice and when it is almost done, add butter into a small sauce pan. Cook on medium heat while adding the flour, salt, and poultry seasoning. Stir to mix and cook for one minute. Begin to slowly add the milk while whisking. This will form a thick and creamy sauce.
Remove the rice and veggies from the oven when done. Add the cut up chicken and the sauce to the casserole dish with the rice and veggies, stirring to combine all the ingredients.
Return the casserole dish to the oven. Increase the oven temperature to 450 degrees baking for 30-20 minutes or until chicken has reached an internal temperature of 165 degrees F.