If using salt cod: Soak salt cod in a large volume of water in the refrigerator for 12 - 24 hours. Change water several times. Place fillet into a 3 -4 quart sauce pan with a large volume of water, bring to a boil and turn heat down so that liquid is simmering. Cook for 10 minutes, remove from heat and leave in water for 10 – 15 minutes. Rinse under cold water and break into flakes.
Boil potatoes in salted water until done, drain and return to pan over moderate heat for 2-3 minutes to evaporate excess moisture. Mash as for mashed potatoes and keep warm. Saute onions or scallions in a small amount of butter until soft.
Mix fish, potatoes, butter and onions to- gether using a fork, then mix in eggs, mus- tard, pepper and herbs, if using.
Form the mixture into cakes and refrigerate for up to 6 hours if not cooking immediately.
Heat enough butter to coat a frying pan, preferably non stick, and cook over medium heat until each side is well browned and the cakes are heated through.
Serve immediately with slices of lemon, tarter sauce or mayonnaise mixed with lemon juice and capers. Smaller cakes can be served as a first course, while larger cakes, served with salad and bread, make a great lunch or light dinner.