via The Spruce Eats
Preheat oven to 350F/180C.
Lightly grease 2 quart baking dish.
Heat butter in a large heavy skillet over medium heat. Add mushrooms and onions and cook until tender and lightly browned, about 8 minutes.
Stir flour into pan. Cook, stirring, 1-2 minutes.
Gradually add milk, stirring, and cook until thickened, 3-5 minutes. Add pepper and salt.
In baking dish, combine cooked potatoes, chicken, and mushroom sauce.
Top with buttered breadcrumbs or shredded cheese, bake 25-30 minutes.