Preheat the oven to 200°C/fan 180°C/gas mark 6. Line the base of a large fluted rectangular baking tray with parchment paper. Lay the pastry on the sheet. Prick the base and bake uncovered for 10 minutes.
Meanwhile, place the salmon on a non-stick baking sheet and bake the fish for 10 minutes. Remove from the oven and break the salmon into small pieces, discarding the skin.
Beat together the three cheese sauce and the eggs. Season well with black pepper.
Remove the pastry from the oven, scatter the watercress over the pastry and pour the sauce on the top. Top with the salmon and cheese and bake for 40-45 minutes, or until golden.