Bring a medium pot, containing enough water to completely cover the sliced potatoes and sliced onions, to a rapid boil.
Parboil for 5 minutes; remove from stovetop; drain well; allow to cool.
Pre-heat oven to 375°F and grease a 9 x 13-inch baking dish.
Spread 1/3 of the potato and onion slices into the bottom of the prepared baking dish.
Sprinkle 3 tbsp. flour over the potato and onion slices.
Arrange 2 tbsp. butter on top of the flour.
Season the entire layer with salt and pepper; the more liberal the salt, the tastier the scalloped potatoes.
Repeat layering twice more.
Heat milk in a saucepan until warm.
Pour enough warm milk over the mixture in the baking dish so the top of the liquid is level with the final layer of potatoes.
Tightly cover the baking dish with aluminum foil.
Bake in pre-heated oven until potatoes for 60 minutes.
Remove from oven, remove aluminum foil and return to oven and bake, uncovered, for an additional 30 minutes.
