Preheat oven to 350 degrees F.
Spray a 2 quart casserole dish with non-stick cooking spray.
Melt the butter in large saucepan. Add flour and whisk together for 1 minute.
Add broth, whisking and allow to come to a boil. Add thyme, salt and pepper as you whisk.
Bring mixture to a boil and continue to whisk occasionally. Lower the heat and add milk and cheese.
Cook for 3-5 minutes until sauce is thickened and milk is warmed through.
Pour sauce over cooked pasta and add diced chicken to the mixture. Stir and pour into the casserole pan.
Cover and bake for 20-25 minutes.