1. Make the sauce ahead of time and refrigerate for a few hours to give the flavors a chance to blend and the mayonnaise time to absorb the color of the curry powder and mustard. Stir together all of the ingredients in a small bowl. Scrape into a serving dish and refrigerate until well chilled.
2. Process the onion finely in the bowl of a food processor. Add the fish, dill, parsley, flour, egg, 2 tablespoons of melted butter, salt, and pepper and pulse until ground medium fine. Transfer the mixture to a bowl and refrigerate, covered, for at least 1 hour and up to 4 hours (the longer you refrigerate the fish mixture, the greener the fish cakes will be).
3. Shape the fish mixture into 8 patties. Set them on waxed paper.
4. Heat the oil and the remaining 1 tablespoon of butter in a heavy, wide skillet over medium heat. When the butter begins to foam, add the fish cakes and cook for about 6 minutes, until golden, periodically sliding a spatula under the cakes to make sure they don’t stick. Carefully flip the fish cakes, lower the heat to medium-low, and continue cooking until nicely browned, about 6 minutes more, periodically using the spatula to keep the fish cakes from sticking. Serve immediately, with remoulade sauce on the side.
Reprinted with permission from Fire + Ice: Classic Nordic Cooking, by Darra Goldstein, copyright © 2015, published by Ten Speed Press, an imprint of Penguin Random House LLC.