Preheat oven to 200C/400F/Gas 6.
Place the plain flour, salt and butter into a food processor and blend until the mixture resembles breadcrumbs. Add the egg and gradually add the water until the mixture comes together into a ball.
With floured hands, place the dough onto a floured surface and roll to the thickness of a one pound coin using a floured rolling pin.
Lightly grease two 8cm/3in ovenproof saucers with butter. Use the pastry to line the saucers, cutting out a circle slightly larger than the saucers, then trim away the edges.
Spread the mustard onto the middle of the pastry, then cover with the sliced potatoes, Camembert and herbs and season well with salt and freshly ground black pepper.
Transfer to the oven and cook for 10-12 minutes, or until the pastry is golden-brown and the cheese is bubbling.
To serve, transfer to a serving plate and garnish with the chives.