My dad used to make this when I was a kid and I always loved it. It's easy and uses things I usually have on hand. Serve over toast points, rice, or noodles.
Sauté mushrooms in 1 tbsp butter until done and set aside.
Melt remaining 1/2 cup butter in large sauce pan and cook onion until softened. Stir in flour and cook for 1 minute to make a rue.
Whisk in broth and bring to a simmer. Add chicken, pimentos, peas, salt & pepper, sherry and mushrooms with any accumulated juices. Let simmer, stirring frequently, until somewhat thickened.
Stir in milk and serve over toast points, rice or egg noodles.