For the pastry case, put the flour and butter into the bowl of a food processor. Add a pinch of salt and blitz until the mixture resembles fine breadcrumbs. If you prefer, rub the butter into the flour in a bowl with your fingertips.
Add the egg yolk and enough water to bring the dough to a firm ball. The less water you add the better, as too much will cause your pastry case to shrink in the oven.
Pat the pastry into a flat round on a floured surface then roll out to a rough circle large enough to line the tart tin. Lightly butter the tin, dust it with a small amount of flour, shake out any surplus then lower in the circle of pastry. Push the pastry right into the corner where the rim joins the base without stretching it. Make certain there are no holes or tears. Trim the overhanging pastry and place in the fridge to chill for 20 minutes.
Set the oven at 200C/400F/Gas 6. Put a baking sheet in the oven to warm. Line the pastry case with kitchen foil and baking beans and slide onto the hot baking sheet. Bake for 15 minutes, then remove from the oven and carefully lift the beans and foil out.
Return the pastry case to the oven for five minutes or so, until the surface is dry to the touch. Remove from the oven and set aside. Turn the oven temperature down to 180C/350F/Gas 4.
For the filling, place the salmon in a baking dish, brush with oil or butter, season lightly with salt and pepper and bake for 15 minutes, or until the flakes part easily. Remove from the oven and leave to cool a little, then break into large pieces. Place the pieces of fish in the tart case.
Wash the watercress leaves and remove and discard the thickest of the stems. Put the still-wet watercress into a saucepan, cover tightly with a lid and place over a high heat. Cook for a couple of minutes, or until the leaves wilt a little. They should keep their colour. Turn the watercress over once with tongs, let it steam a further few seconds, then remove from the heat, cool under cold running water and squeeze dry.
Tuck the watercress into the tart case in between the salmon pieces. Gently whisk the eggs and cream, pour into the case and bake for 25 minutes, or until set. Serve warm.