For the filling clean, wash and finely dice the mushrooms. Peel and chop the onion. Sweat in 1 tablespoon of hot oil, let it cool down on a plate. In the meantime cut the plaice fillet into small pieces.
Put the salt, cream and egg yolk in a high mixing bowl and puree finely with the cutting knife of the hand mixer. Then cool for 10 minutes in the freezer. Wash dill and parsley, dab dry and chop.
Mix the prepared ingredients and breadcrumbs. Cool again. In the meantime, wash the plaice, dab dry. Cut the plaice on the light side of the skin along the fishbones. Carefully loosen the fillet on the left and right side, open it up, do not separate it.
Pour prepared farce into the opening. Sprinkle the plaice with lemon juice. Put them on a baking tray coated with remaining oil. Cook in the preheated oven (electric cooker: 200 °C/ gas: level 3) for about 20-25 minutes.
Cover with aluminium foil in between. In the meantime, peel the potatoes, cook them in salted water for about 20 minutes, drain them and shortly before serving, toss them in hot fat. Warm up the hollandaise sauce over medium heat while stirring.
Arrange everything together with herbs, lemon wheels and mushroom slices garnished on preheated plates.