Boil kippers in 400ml milk, then cool, drain, and flake the fish while removing bones.
Mash cooked potatoes and combine with flaked kippers, parsley, egg yolk, salt, and pepper.
Form the mixture into four patties and chill in the refrigerator for two hours.
Coat each patty in flour, then beaten egg, and finally panko breadcrumbs.
Fry fishcakes in olive oil over medium heat for three minutes per side until golden.
Blend egg yolks, vinegar, mustard, and salt, then slowly stream in rapeseed oil to thicken.
Stir cornichons, capers, parsley, chives, and lemon juice into the sauce and season.
Toss watercress with olive oil, salt, and pepper in a bowl.
Serve fishcakes with tartare sauce, watercress salad, and lemon wedges.