Sift the flour and salt into a large mixing bowl. Add the butter cubes and use your fingertips and thumbs to lightly rub it into the flour until the mixture resembles breadcrumbs. Add the Parmesan (if using) and rub again until the cheese is mixed in evenly. Now add the beaten egg yolk and water, and use a round-bladed knife to combine the wet ingredients with the dry until the pastry comes together. Gather it up with your hands and knead very briefly into a ball on a lightly floured surface. Try not to handle it too much at this stage, or the fat will get warm and the pastry will become tricky to use and may turn out tough and chewy. Wrap in clingfilm and chill for 30 minutes before use.
On a lightly floured work surface, roll out the pastry to about 3mm thick and line your tart tin with it, making sure the edge of the pastry stands just a little proud above the rim. Trim the edges, then prick the base with a fork and return it to the fridge for 30 minutes. Don’t be tempted to skip this step – it helps stop the pastry shrinking in the oven.
Preheat the oven to 200°C/180°C/gas mark 6 and put a baking sheet in to heat up. Tear off a sheet of foil big enough to enclose all your salmon fillets and lay it on a work surface. Put the salmon in the middle, add the lemon juice and some salt and pepper, and scrunch to seal tightly. Bake in the oven for 12 minutes, then remove and set aside to cool.
Take the pastry case out of the fridge, line with crumpled non-stick baking paper and fill with baking beans. Place in the oven on the hot baking sheet and bake for 20 minutes. Then remove the paper and beans and return the case to the oven for a further 5 minutes. This will allow the pastry base to dry out before you pile in the filling.
Reduce the oven to 180°C/160°C fan/gas mark 4. Melt the butter in a frying pan over a medium heat. Add the onion and let it soften for 8 minutes. Remove from the heat and leave to cool, then spoon into the pastry case. Thickly flake the salmon (discarding the skin) and scatter it over the onions with the parboiled potatoes and most of the chopped watercress and dill.
Whisk together the eggs and cream, season well with salt and pour into the pastry case. Scatter with the remaining watercress and dill and the peppercorns. Place on the hot baking sheet in the oven and bake for30 minutes or until the filling has just set and the top is turning golden.