Step 1

Preheat oven to 375 degrees F (190 degrees C). Grease a 9x13-inch baking dish.

Step 2

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.

Step 3

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.

Step 4

Bake in preheated oven until potatoes are tender, 45 to 60 minutes.