Original recipe says to cook chicken with onion, celery & carrots for flavor. I have never done this, but include it in case you wish to do so. The original recipe also said to cook the frozen broccoli, but I find that if I thaw it out completely, then pat it dry, there is no need to cook it.
Chicken Divan https://www.thegratefulgirlcooks.com/chicken-divan/