01 -
Preheat the oven to 375°F (190°C). Grease a 9x13-inch baking dish.
02 -
Spread about 1/3 of the potato slices into the bottom of the prepared baking dish. Top with about 1/3 of the onion slices. Sprinkle 3 tablespoons flour over the potato and onion. Arrange 2 tablespoons butter atop the flour. Season the entire layer with salt and pepper. Repeat layering twice more.
03 -
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.
04 -
Bake in the preheated oven until potatoes are tender and the creamy sauce is bubbling with a golden brown crust on top, 45 to 60 minutes. The cream sauce may bubble over the sides of the baking dish; place a baking sheet underneath to catch any drips.