Rinse fish fillet, sprinkle with juice of one lemon and season with salt. Clean and rinse the leek. Cut prepared vegetables into narrow strips.
Blanch in boiling salted water for about 3 minutes and drain well. Rinse dill, chop coarsely and mix with vegetables. For the sauce, melt fat and sweat flour in it. Add clear soup and whipping cream while stirring continuously and let it boil. Season with pepper, saffron, salt, juice of one lemon and horseradish. Grease a 20×20 cm gratin dish. Pour in some sauce, place four lasagne sheets and sauce on top. Cut the fish fillets into pieces and place them in the dish. Layer the ingredients as described. Finish with the sauce. Bake in a heated oven (electric stove: 200 degrees /
gas stove: level 3)