Heat 2 tablespoons of the butter and sauté the scallions in it for about 5 minutes, stirring frequently. Do not allow the scallions to brown. Set them aside.
Meanwhile, cook the potatoes, and when they are soft, drain and mash them with 1 tablespoon of the butter and the sour cream. Combine them with the fish and set aside.
Beat the egg, adding the mustard, a generous amount of salt, and black pepper. Stir in the scallions and parsley. Combine this mixture with the potatoes and fish. (A spatula works well. Do not use a mixer, or the potatoes will become gummy.)
Form the fish mixture into 8 cakes about 2 1/2 inches in diameter and dip each cake into the bread crumbs. Heat the remaining butter and the oil in a large frying pan, and sauté the fish cakes slowly for about 3 minutes on each side, or until they are lightly browned and cooked through. Serve with ketchup on the side.Makes 4 Source: