Step 1

Preheat oven to 375 degrees F (190 degrees C).

Step 2

Thoroughly grease a 9x13-inch baking dish with either butter or cooking spray.

Step 3

Thinly slice potatoes and onion and pat dry.

Step 4

Dice the butter and split it into three portions.

Step 5

Cover the bottom of the baking dish with a layer of potatoes and top with 1/3 of the onions.

Step 6

Sprinkle three tablespoons of flour evenly over the potatoes and onions.

Step 7

Distribute 1/3 of the diced butter over the flour evenly.

Step 8

Repeat these layers twice more.

Step 9

Gently heat the milk on the stove until warmed through.

Step 10

Carefully pour the milk into the dish, to cover the potatoes and onions. It should fill the dish to the top of the potatoes.

Step 11

Bake, covered in foil, for 45 minutes.

Step 12

Remove the foil and bake for a further 10 to 15 minutes, until the potatoes are fork-tender.