Preheat the oven to gas 6, 200°C, fan 180°C. Lightly flour a clean surface and roll out the pastry thinly. Using an 8cm (3.5 inch) fluted cutter, stamp out 12 rounds and press into a greased 12-hole bun tin. Prick each base with a fork and chill for 10 minutes.
Line each pastry case with a small piece of kitchen foil (allow a slight overhang to help you remove it later) and fill with baking beans. Blind bake in the oven for 10 minutes, until the pastry is just turning golden and firm to the touch. Remove the foil and baking beans, then return to the oven for a further 3 minutes, until golden and crisp. Turn down the oven to gas 4, 1380°C, fan 160°C.
Beat together the cream, egg and horseradish with a little salt and fresh black pepper in a jug. Divide the flaked salmon between the pastry cases. Blanch the watercress for at least 15 seconds in boiling water and allow to cool and tuck in a few sprigs of watercress around the flakes in the pastry cases. Carefully pour in the egg mixture around the salmon and watercress, until just full.
Bake in the oven for 12-15 minutes, until the filling has just set. To serve, remove from the tin and enjoy warm or allow to cool.