4 tablespoons butter
2 pounds white flaky fish, cut into serving pieces
2 tablespoons fresh lemon juice
½ teaspoon salt
2 medium-sized onions, peeled, thinly sliced and separated into rings
3 tablespoons tomato puree
1 tablespoon white wine vinegar or cider vinegar
½ teaspoon grated fresh horseradish or 1 teaspoon bottled horseradish, thoroughly drained and squeezed dry in a towel
2 medium-sized dill pickles, cut lengthwise into thin wedges
1. Preheat the oven to 375°F. With 1 tablespoon of the butter, coat the bottom and sides of a shallow baking dish or casserole large enough to hold the fish in a single layer. Set the dish aside. Spread the fillets on wax paper, sprinkle them with lemon juice and salt, and let the fillets marinate for 10 minutes.
2. In a heavy 8- to 10-inch skillet, melt 2 tablespoons of butter over moderate heat. When the foam subsides, drop in the onion rings and cook them, turning them frequently, for 5 minutes, or until the rings are soft and transparent but not brown. .
3. Arrange the fish fillets side by side in the prepared baking dish. Beat the tomato puree, vinegar and horseradish together in a bowl, and spread the mixture evenly over the fillets. Scatter the onion rings and pickle wedges over the fish.
4. Cut the remaining 1 tablespoon of butter into small pieces and dot the fish with them. Bake in the middle of the oven for about 15 minutes, or until the fillets are opaque and firm to the touch. Do not overcook. Serve at once, directly from the baking dish.