How to Make It
In a large saucepan, combine all of the ingredients and bring to a boil. Simmer over moderately low heat until the vegetables are just softened, about 12 minutes. Let cool completely.
Meanwhile, in a large skillet, heat 2 tablespoons of the oil until shimmering. Working with half of the fish at a time, season the pieces with salt and dust with flour; pat off the excess. Add the floured fish to the skillet and cook over moderate heat, turning once, until golden and just cooked through, 5 to 6 minutes. Transfer the fish to a plate. Repeat with the remaining 2 tablespoons of oil and fish.
Spoon half of the brine and vegetables into a large bowl or baking dish. Add the fish and pour the remaining brine and vegetables over the top. Cover with plastic wrap and refrigerate overnight. Bring the fish to room temperature before serving.
Transfer the pickled fish and vegetables to a platter. Garnish with dill sprigs and serve with rye bread, crème fraîche and minced red onion.