Cut puff pastry into 2 equal size pieces slightly larger than your salmon. Place 1 piece of pastry on oiled or parchment paper lined cookie sheet and set side of salmon on top. Sprinkle with parsley. Lay sliced black truffles over salmon like scales trying to cover entire fish. Sprinkle the chopped hard boiled eggs over truffles and season with salt and pepper.
Place other piece of pastry on top and seal edges and crimp with your fingers. Brush with egg wash and bake until pastry is golden brown, about 40 to 45 minutes. Let cool for 10 minutes and then slice into 6 equal size pieces, garnish platter/plate with watercress and serve.