Boil plenty of water. Add the lasagne plates and cook for 3-5 minutes. Remove with a skimmer and place in a bowl of cold water. Peel and finely dice the onions.
Clean, wash and quarter the pointed cabbage and cut out the stalk. Cut the cabbage into strips. Steam onions and pointed cabbage in hot oil for about 10 minutes. Wash the redfish fillet, dab dry, cut into cubes and add to the pointed cabbage 5 minutes before the end of the cooking time.
Season to taste with salt and pepper. Remove the lasagne sheets from the water, dab dry and fold lengthwise. Fill the lasagne plates with pointed cabbage and fish and place them side by side in a greased lasagne dish.
For the béchamel sauce melt the fat in a pot, dust with flour and let it sweat on. Deglaze with broth and cream while stirring and bring to the boil. Season with salt, pepper, lemon juice and sugar.
Wash the dill, dab dry, chop and add to the sauce together with the capers. Leave to swell for 10 minutes at low heat. Pour the sauce over the lasagne and bake in a preheated oven (electric range: 200 °C/ gas: level 3) for approx. 20 minutes.