· 3 pounds potatoes, thinly sliced· ½ 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
Step 1Preheat oven to 375 degrees F (190 degrees C). Grease a 9x13-inch baking dish.Step 2Spread 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 3Heat 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 4Bake in preheated oven until potatoes are tender, 45 to 60 minutes.