To make the pastry sift the flour and salt into a large bowl, rub in butter until mixture resembles breadcrumbs. Stir in the water, a little at a time, until a smooth dough is formed. Turn onto a floured surface, lightly press into a flattened ball, wrap in clingfilm and refrigerate for an hour.
Turn onto a floured surface and roll out to a thickness of approx. 4mm. Cut pastry circles and line eight 12cm tartlet tins. Place a layer of greaseproof paper in each, add a few baking beans, bake for 15 minutes, remove the paper then allow to cool.
When cool, sprinkle the parmesan into the pastry cases. Mix the remaining filling ingredients together and pour into the cases. Cook for 30 minutes, sprinkle the reserved gruyere cubes on the top and cook for a further 5 minutes.