In a large bowl, combine the cooked chicken, sliced potatoes, cream of chicken soup, milk, garlic powder, salt, and pepper. Mix well.
Pour the mixture into a greased baking dish.
If using breadcrumbs, mix them with olive oil and sprinkle on top of the casserole.
Bake for 45-60 minutes, or until the potatoes are tender and the top is golden brown.
Let cool for a few minutes before serving.
Notes
Refrigerate leftovers in an airtight container for 3 to 4 days. To freeze, assemble unbaked, wrap tightly, and freeze up to 3 months; thaw overnight before baking. For a homemade substitute for cream of chicken soup, use a white sauce made with milk, flour, and chicken stock.