Rinse and drain the lenses. Peel and chop the onions. Heat butter in a pot. Sauté onions in it. Add lentils, bay leaf and approx. 400 ml water. Bring to the boil with broth and simmer covered for about 10 minutes.
Peel or clean and wash the soup greens. Cut carrots into slices, celery into strips, leek into rings. Add vegetables to the lentils and continue cooking for about 10 minutes.
Stir the cream into the lentil vegetables and simmer for about 5 minutes. Squeeze lemon. Season the lentils with salt, pepper and 1 tsp lemon juice.
Wash the fish, dab dry and cut into pieces. Sprinkle with the rest of the lemon juice and salt. Heat oil in a coated pan. Fry the fish fillets for 2-3 minutes on each side. Season with salt and pepper.
Arrange lentil vegetables and zander on plates.