Grease a baking dish, and preheat the oven to 400F. Peel the potatoes, do a medium slice on your potatoes. Slice the onions thinly.
Layer the potatoes, onions, flour, salt and pepper and butter, about 3 times. Heat the milk, pour the milk over the top of the dish.
Cover with foil, and bake until potatoes are tender (about 50 minutes).
Uncover, add the cheese and bake until top browns, then remove from the oven, and let it stand while the sauce thickens.
Devour (and hide the leftovers for yourself).
A mandolin will help you get super even slices. If you don’t have one, a super sharp knife is essential.
Make ahead and bake then reheat when you need it. Or you can make it to the point of baking, and refrigerate overnight if you’d like.