Bring the potatoes to the boil, adding a pinch of salt. Turn down heat and cook for 20 minutes. Drain and mash thoroughly to get rid of lumps. Add 4 tablespoons butter, salt and freshly ground pepper.
Combine the oil, vinegar, salt and pepper in a small bowl to make a marinade. Dip the fish in it and wrap each piece in foil, with a sprig of parsley. Put parcels in fish kettle or on baking sheet. Add boiling water and steam for 10 to 12 minutes until the juices flow. Remove from silver foil and check for bones. Flake it into big chunks and combine with the mashed potatoes in a big bowl.
In a frying pan, saute the chopped shallot, scallions, chives, parsley, fennel, dill, garlic and chile in 4 tablespoons butter and olive oil until soft. Combine with potatoes and fish. Cover with cling-film and leave to chill in refrigerator for 30 minutes, see Cook's Note*.
Remove and form the fishcakes using a dessertspoon and pat them into shape. Dust with flour. Heat sunflower oil in frying pan until very hot and fry the cakes for 3 or 4 minutes on each side until golden brown. Dry them on paper towels.
Garnish with parsley and slices of lemon.
Ratio of potato to fish should be 50:50. Leaving the mixture to chill makes it easier to handle.