Melt the butter in a large frying pan, add the onion and leeks first, and then the potatoes after a few minutes, cook until soft.
Add the flour and then the stock and stir together well. Turn down the heat and add the crème fraiche, mustard and ham. Season to taste and the add ingredients to a pie dish.
Dust a little flour on the surface and roll out the puff pastry, add it to the top of the pie and trim the edges. Use a fork to press down the edges and cut a few slits to allow air to escape. Brush the pastry will egg and cook for 30 minutes until golden (220C/Fan 200C/Gas 7).
The pie may be served with any extra vegetables from your box such as spring greens, carrots or savoy cabbage.