Clean, sour, salt the fish fillets. Oil 4 rectangles of aluminum foil, spread the fillets on top. Mix the tomato paste and horseradish (use more if you like), mix in the diced onion and spread on the fish. Put flakes of butter on top and close the foil packets tightly. Cook in a preheated oven at 200 degrees for about 20 minutes. Serve sprinkled with fresh dill if you like. If you like, you can also use fresh tomato slices.