Clean, rinse and peel carrots, celery and kohlrabi if necessary. Cut everything into very fine strips 4 cm long. Clean leek, rinse thoroughly and cut into slices. Stir clear soup with 3 tbsp hot water. Mix with the vegetables.
Rinse dorado, dry, season with salt, season with pepper and sprinkle with juice of one lemon. Cut the roasting tube 20 cm longer than the fish. Tie one side with a strip of foil or thread. Put the vegetables inside. Spread butter flakes evenly on top. Place fish on top of vegetables. Close the roasting tube, pierce it a few times with a needle or a fork and place it on the cold baking tray. Place in the oven heated to 175 degrees (lowest shelf) and cook for about 45 minutes.