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.
"Vegetables are perishable, so we don't have any indication of what they looked like 500 years ago," says James Nienhuis, a professor in the Department of Horticulture at the University of Wisconsin.