Preheat oven to 425°F. Peel the potatoes and slice them 1/8" thick. Place the slices in a bowl of cold water and set aside.
Rub the inside of a 10" pie plate (about 2" deep) with 1 tablespoon of the butter, coating all inner surfaces. Drain the potatoes and pat dry with a kitchen towel. Layer about half of the potato slices in a spiral pattern on the bottom of the plate. Sprinkle the layer with half the salt, pepper, sage, butter and cheese.
Layer the remaining potatoes on top. Depending on the size of your baking dish, a third layer of potatoes may be necessary. Spread the rest of the seasoning, butter and cheese on this layer. Carefully pour the boiling milk over top of the potatoes and place in the upper third of the preheated oven.
Bake for 25-30 minutes. Keep an eye on them while baking; if the top layer of potatoes is browning too much, cover with foil. In the meantime, slice the apples into 1/4 to 1/2" thick slices (peel on or off is up to you).
Remove potatoes from oven after 25-30 minutes and check doneness - a butter knife inserted should meet no resistance from the potatoes. If the potatoes are done, layer the apples on top and return to the oven for another 5 to 7 minutes.