Cut the fish into chunks and then place them in food processor with salt, tamari sauce and pepper to taste. Lightly blend but do not process too much.
Boil whole potatoes for 5-10 minutes. Cool and grate coarsely and add to fish. Chop the fresh coriander and add to the fish and potato mix. Add some egg white to bind.
Using wet hands, shape into four large patties or 8 smaller ones. Make sure they are not too thick.
Lightly dip fish cakes into beaten egg and coat in corn flour. Lightly spray pan with oil and cook the fish cakes over a medium heat for about 5-7 minutes on each side until they are brown and cooked inside.
For the hollandaise sauce, blend the egg yolks, white wine vinegar, Dijon mustard and salt in a food processor. While the motor is running, gradually pour the oil into the food processor in a steady stream. Bring pot of water to the boil. Transfer mixture to a small err pot and place over pot of boiled water.
Whisk continuously until mixture is lightly cooked and thickens to mayonnaise consistency. Do NOT overcook! Stir in the cornichons, capers, parsley, chives and lemon juice. Season, to taste, with salt and freshly ground black pepper.
For the watercress salad, place the watercress in a bowl, add the oil and season, to taste, with salt and freshly ground black pepper. Mix together until well combined.