Blitz flour, butter, and salt in a food processor until the mixture resembles fine breadcrumbs.
Add egg yolk and enough water to form a firm ball of dough.
Roll pastry out on a floured surface and line a buttered 24cm tart tin.
Trim the edges and chill the pastry case in the fridge for 20 minutes.
Preheat oven to 200°C and warm a baking sheet inside.
Line the tart with foil and baking beans, then bake on the hot sheet for 15 minutes.
Remove beans and foil, then bake for 5 more minutes until the surface is dry.
Lower the oven temperature to 180°C.
Bake the seasoned salmon for 15 minutes, then flake into large pieces and place in the pastry case.
Steam wet watercress in a lidded pan for 2 minutes, squeeze dry, and tuck between salmon pieces.
Whisk eggs and cream together, pour into the case, and bake for 25 minutes until set.