Peel the potatoes and slice them thickly. You’ll want about 1 kg (4 large potatoes), cut into chunky slices.
In a large bowl, combine the flour, salt, and plenty of freshly ground pepper. Mix well.
Add the sliced potatoes to the bowl and toss to coat them generously in the flour mixture. Discard any leftover flour.
Generously butter a casserole dish (with or without a lid).
Arrange the floured potato slices into the dish, overlapping them slightly to fit snugly.
In a saucepan, heat the milk and butter together until steaming, almost boiling. Remove from heat as soon as you see steam rising and most of the butter has melted.
Pour the hot milk and butter mixture evenly over the layered potatoes.
Cover the dish with a lid or foil (sprayed with cooking spray if using) and bake in a preheated oven at 170°C (340°F) with fan, or 190°C (375°F) without fan, for 1 hour.
After 1 hour, remove the lid or foil and return to the oven for 15–20 minutes, increase oven temperature to hot and cook until the top is golden.
Serve hot as a comforting side or simple vegetarian main. Leftovers reheat beautifully.