Sift the flour and salt into a bowl

Add the butter incorporating using your fingertips or pastry blender until it resembles fine breadcrumbs

Add just enough cold water to make the pastry form a ball

Put the pastry ball into a plastic bag (or wrap in cling film/Saran wrap), press out the air and refrigerate for half an hour

Peel the potatoes and onion, slice the potatoes a little thicker than a pound coin, and the onion into half rings

Parboil the potatoes until just soft but still holding their shape (8-10 minutes)

Sweat the onions and thyme over a low heat in the butter until soft. Stir regularly so that they don't stick to the base or colour

Roll out two thirds of the pastry and use to line a 20cm/8inch diameter round pie dish

Trim the edges using a sharp knife around the circumference

Drain the potatoes into a colander/large sieve and allow to cool

Once cool, layer the potatoes, onions and butter curls into the lined pie dish. Season with salt and white pepper

Using a pastry brush, brush around the edge of the pastry with the milk

Roll out the remaining third of the pastry (also using the trimmings from the base if necessary) and carefully lift it over

Using your fingers or a fork, press the edges of the top & bottom layers of the pastry together

Remove any excess pastry lid with a sharp knife

Make a few holes in the pie top to allow steam to escape

Brush the remaining milk over the top of the pie

Bake at 180ºC/350ºF/Gas mark 4 for 30 minutes until golden brown on top

Slice & serve immediately