Peel, wash and thinly slice the potatoes. Heat the oil in a pan and fry the potatoes in it at medium heat while turning for about 20 minutes. Season with salt and pepper. Put fish stock and wine in a pot, bring to the boil and let it boil down for 10-12 minutes at low heat. Wash the fish, dab dry and cut into 8 slices.
Sprinkle with lemon juice and season with salt and pepper. Meanwhile heat 25 g butter in a pan and fry the fish slices in it at medium heat for 4-5 minutes on each side. Season again with salt and pepper. Meanwhile peel and finely chop the shallots. Heat the rest of the butter in a pan and sauté the shallots in it. Dust with flour, sweat and deglaze with the reduced fish stock while stirring. Bring to the boil and season to taste with mustard, salt and pepper. Remove the pot from the heat. Add egg yolk and stir in. Do not boil any more.
Bring to the boil and season to taste with mustard, salt and pepper. Remove the pot from the heat. Add egg yolk and stir in. Do not boil any more. Whip cream lightly and stir into the sauce. Arrange fish, fried potatoes and sauce on plates and serve garnished with lemon wedges and dill as desired. Cucumber salad tastes good with it