Clean, peel and wash the celery. Peel and wash the carrot. Cut vegetables into thin strips. Clean, wash and cut leek into thin slices. Wash herbs, shake dry, pluck leaves from the stalks and chop or cut into fine rolls.
Wash the fish, dab dry and season with salt, pepper and lemon. Sprinkle the herbs on the skin side of the fish fillet, cut in half lengthwise and roll up. Melt the fat in a pot. Sauté the vegetables for about 1 minute, pour on the stock, place the fish rolls on the vegetables and steam in a closed pot for about 10 minutes.
Meanwhile cook the gnocchi in boiling salted water for about 4 minutes until they float on top. Arrange vegetables, fish and gnocchi on a plate.