Defrost the fish. Peel, wash and slice the potatoes. Line a baking tray with baking paper and lightly coat with 1 tablespoon of oil. Spread potato slices on the baking tray and sprinkle with coarse salt.
Roast in the preheated oven (electric cooker: 200 °C/ circulating air: 175 °C/ gas: level 3) for approx. 30 minutes. In the meantime, wash the fish, pat dry, sprinkle with lemon juice and pat dry again. Season with salt and pepper.
After 15 minutes, place the fish on the baking tray with the potatoes and cook it. In the meantime peel and finely dice the shallot. Heat 1 tablespoon of oil in a pan and sauté the shallot in it. Deglaze with wine and let it boil down for about 2 minutes.
Add vegetable stock, bring to the boil and let it boil down for about 10 minutes. Pluck the marjoram leaves from the stalks, wash, dab dry and chop finely. Mix potatoes and marjoram. Arrange fish, potatoes and sauce on plates.
Serve garnished with some lemon peel.