Preheat Oven: Preheat to 180°C (350°F). Lightly grease your pie dish.
Prepare Pastry: Line the dish with shortcrust pastry. Trim excess and prick the base with a fork.
Slice Potatoes: Use a sharp knife or mandoline to slice potatoes evenly for even cooking.
Layer Filling: Alternate layers of potatoes, onions, butter cubes, salt, and pepper until all ingredients are used.
Add Milk: Pour milk slowly over the layers to evenly soak the potatoes.
Seal Pie: Cover with pastry, crimp edges, and cut a small slit in the center for steam.
Bake: Bake for 60–70 minutes until the pastry is golden and the filling is tender.
Rest and Serve: Let rest 10 minutes before slicing and serving.