Wash the fish fillets, dry and lightly touch the skin, then salt. Heat the butter and olive oil in a pan and burn the fillets everywhere. Reduce heat and cook the screed for 3-4 minutes. It should still be a bit glassy at the core. The cooking time depends on the thickness of the fillets. When preparing, add 1 sprig of rosemary and the light yellow stalks from the middle of the celery.
Ingredients for beurre blanc:
Peel and halve the shallot. Cut the butter into small pieces or cubes and cool. Put the vegetable stock, wine (Pinot Gris, dry Riesling or Riesling Spätlese to taste), shallot and bay leaf in a saucepan and reduce the liquid by a third. Then add cream and greatly reduce the heat. The sauce should hardly boil. Gradually add the pieces of butter. Stir constantly with a whisk so that the butter blends well with the liquid and the sauce becomes nice and creamy.
If the sauce is still too mocking after adding the butter, simply add the flour butter. Knead some room temperature butter with the same amount of flour. Gradually add the flour until the desired consistency is reached. Finally, season with a little peel of the lime and salt.
Ingredients for the potato purity:
Peel a squash, grate it and boil in salted water. Drain the water and chop the potatoes into a thick mass. Add some hot broth, salt and nutmeg. You can also cook the potatoes in broth immediately. Then, while draining, reserving the broth, chop the potatoes and add broth until the desired consistency is reached.
Place the potatoes on a plate and place the fillets on top. Pour the sauce over everything. Garnish with celery and rosemary.