Boil the prawns in the water for 3 to 4 minutes, until they turn pink. Remove and let cool. Slice the prawns into small chunks and mix with the mayonnaise and black pepper.
Lay the Sesame Omelet on a flat work surface and spread the prawn mixture across the center of the omelet. Sprinkle the fish sauce over the prawn mixture, and place the watercress sprigs in