For the potato bread, mix the potato, flour and egg yolk together in a bowl. Season with sea salt and freshly ground black pepper and bring together to form a dough.
Lightly flour a work surface and roll out to 5mm/¼in thick. With a pastry cutter, cut out 9cm/3½in rounds.
Heat a dry, non-stick frying pan over a medium heat, add the potato bread rounds and cook for 1½ minutes on each side. Remove the potato breads from pan and wipe the pan dry.
Add the butter to the pan and fry the breads until crisp on each side. Remove from the pan and keep warm.
For the butter sauce, heat the shallot, white wine and white wine vinegar in a pan over a medium heat, and cook until the liquid has reduced in volume until only a tablespoon of liquid remains.
Whisk in the cold diced butter one piece at a time. Remove the pan from the heat, add the capers, anchovy and parsley and season, to taste, with sea salt and freshly ground black pepper.
For the salmon and eggs, place the eggs in boiling water for 4-5 minutes until soft boiled. Transfer them to a bowl of iced water for one minute to stop the cooking process, then peel and keep warm.
Place the salmon in a hot pan with a little oil. Cook for one minute on each side - the salmon has already been cured so it won’t take long. It should still be dark pink in the middle.
To serve, place the potato bread onto serving plates with the salmon on top and gently break the salmon to expose the inner flesh. Place the egg on top of the salmon then cut it into four and season with sea salt and freshly ground black pepper.
Spoon the butter sauce over the egg and salmon, and top with watercress.