Blend butter in a food processor for at least 8 minutes, until it turns white and creamy. At the beaten egg, followed with the water.
Sieve in the flour and salt, and blend together until you get a ball of dough. Do not overwork the dough, and the dough will be hard and chewy.
Turn the dough to a table and roll the dough to a 1/4 inch thickness. Chill in the refrigerator for 15 minutes, then cut into 1 1/2 inch squares. Prick holes into the squares, and freeze for another 1 hour.
Preheat oven 180 C and baked the frozen squares for around 12 mins or until golden brown.
Heat the cream on a low fire, and blend the herbs in the cream until fine. Melt the gelatine with the water in a double boiler until the crystals are dissolved. Then combine the gelatin with the herb cream, season with salt and pepper. Remove from fire and strain with a sieve. Line a 9 inch square tray with saran wrap. Pour the cream into the tray and chill for one hour, then cut the gel in small circles, slightly smaller than the tart.
Place a herb mousse circle on top of the tart, and top it with a small square slice of smoked salmon. Garnish with salmon roe or caviar, edible flower or some green.