Remove the bones from the fish and rinse. Finely chop the onion, the white part of the leek and the celery and simmer until transparent. Add fish bones, 1 piece of lemon and a little lemon juice and fill with white wine and water. Salt and bring to a slow boil. When the broth boils, remove it from the stove and let it stand for about 20 minutes. Then pass the extract through a fine sieve and reduce slightly. Add heavy cream, mix in 3 types of mustard and season with salt and sugar. Finally, bind with cornstarch.
Ingredients for Fried Potatoes:
Peel the onions and chop them finely. Cut the boiled potatoes (preferably from the previous day) into slices. Heat the fat and fry the potato slices over moderately high heat until they are nicely browned. Add onion and season with salt and pepper. If you wish, you can also season with smoked salt or oil.
Ingredients for Spinach:
Wash the spinach well. Peel the onion and garlic and chop finely. Saute onion cubes and garlic in butter. Add the spinach and season with pepper, salt and nutmeg. Stir until the spinach is wilted, adding a little water if necessary.
After washing and removing the bones, dry, season with salt and lemon and coat the pan with flour to prevent sticking. Cook in frying fat over moderately high heat until the roast side is lightly browned, then flip and simmer.