Place the lentils on a sieve, rinse and drain. Peel and chop the onions. Heat butter in a pot. Fry the onions in it. Add the lentils, bay leaf and stock, bring to the boil and cook over a low heat for about 10 minutes.
Peel or clean and wash the soup greens. Cut carrots into slices, celery into strips, leek into rings. Add the vegetables to the lentils and continue cooking for about 10 minutes in a closed pot. Add cream and sparkling wine to the vegetables and let it boil down for about 5 minutes at low heat.
Season to taste with salt, pepper and lemon juice. Wash the fish, dab dry and cut into pieces. Heat oil in a pan. Fry the fish fillets for 2-3 minutes on each side. Season with salt and pepper.
Arrange lentil vegetables and zander on plates.