Peel the potatoes and slice thinly. Arrange a layer of the potatoes in the casserole and top with a layer of onions. Arrange a layer of the ham strips over the potatoes.
Whisk together the flour, salt and pepper. Sprinkle a small amount over the potatoes and dot with butter.
Arrange another layer of potatoes, onions, ham and flour mixture and dot with butter. Continue to layer until all the ingredients are used. Dot the top of the casserole with butter.
Heat the milk in the top of a double boiler over boiling water or in a sauce pan over low heat until small bubbles show around the edges of the pan. Do not let the milk boil. Pour the hot milk over the mixture in the casserole.
Cover with foil and place on center rack of oven.
Bake for 45 minutes. Remove foil and bake an additional 15 minutes or until the potatoes are tender when tested with a fork.