- 3 pounds potatoes, thinly sliced
- 1/2 onion, thinly sliced
- 9 tablespoons all-purpose flour, divided
- 6 tablespoons butter, diced and divided
- salt and ground black pepper to taste
- 3 cups whole milk, or as needed
- Preheat oven to 375 degrees F (190 degrees C). Grease a 9×13-inch baking dish.
- 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.
- 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.
- Bake in preheated oven until potatoes are tender, 45 to 60 minutes.