In a large skillet, saute onion and garlic in butter over medium heat until tender.
In a jar, combine flour and milk. Seal and shake to combine.
Add milk mixture to onion mixture and cook for 5 minutes, stirring constantly. Sauce should thicken.
Mix together noodles, white sauce, broth, sour cream and chicken. Salt and pepper, to taste. Pour into a greased baking dish.
Bake covered at 350 degrees F for 30 minutes, if desired.