Marinate the pikeperch fillets with lemon juice, salt and pepper. Melt the butter in a pan or shallow casserole dish and lightly sweat the onions and mushrooms without letting them take their color.
Add the pikeperch fillets, pour the Riesling over them and cover and cook slowly and carefully over a low heat for about 5 minutes, depending on the thickness of the fillets. Take the fillets out of the pan and place them warm on a plate.
Add the cream to the Riesling stock (remove 1 tablespoon beforehand for the egg yolk) and let it boil down a little. Mix the egg yolk with 1 tablespoon of cream and stir into the sauce, but do not let it boil anymore. Season to taste with salt and pepper, add chopped parsley and pour over the pikeperch fillets. Serve with parsley potatoes.