Boil the potatoes in salted water for 8-10 mins, until tender but still firm. Leave to cool, then slice.
Heat the oven to 200C/180C fan/ gas 6. Unroll the pastry with its baking parchment on a baking tray. Reserve 1 tbsp of the crème fraîche, then spread the rest over the pastry, leaving a 2cm border around the edge. Season. Layer the potatoes, ham and cheese over half of the pastry, seasoning the potatoes as you go.
Fold over the other half of the pastry and use a fork to seal the edges. Trim with a knife for a neat finish – the pie should look like a big sausage roll. Mix a little water into the reserved crème fraîche, then brush all over the pastry. Make a few cuts along the length, then bake for 35-40 mins until golden. Leave to cool for 10 mins, then serve with a salad, if you like.
