For the puree, peel, wash and chop the potatoes and parsnips. Cook in salted water for about 20 minutes.
Clean and wash the leek and cut into fine rings. Wash and halve the tomatoes. Wash dill, shake dry and cut finely.
For the sauce, melt 180 g butter and remove the pot from the heat. Mix the egg yolks in a metal bowl with wine, 1 pinch each of salt and sugar. Whisk in a hot water bath (60-70 °C) with a whisk for 5-8 minutes until creamy.
Take the bowl out of the water bath. Stir the liquid butter first drop by drop, then in a thin stream into the egg yolk cream until a creamy sauce is formed. Add the dill. Season to taste with salt and pepper and keep warm in a bain-marie.
Drain the potatoes and parsnips. Press both through the potato press. Stir in milk and 2 tablespoons of butter. Season with salt and keep warm.
Rinse lemon hot, grate dry and cut into 4 slices. Rinse fish fillets cold, dab dry. Heat oil in a large pan. Fry fish on the skin side for approx. 3 minutes. Turn carefully, fry for about 2 minutes more.
Add lemon slices and fry them. Season fish with salt and pepper. Take it out and keep it warm for a short time. Swivel leek and tomatoes in the frying fat for 2-4 minutes. Serve with zander, mashed potatoes and hollandaise sauce.