Peel and wash the potatoes and possibly cut them in half or quarters. Boil potatoes in salted water for about 20 minutes. Peel and wash carrots and slice them into long, thin strips with a potato peeler or on a vegetable slicer. Peel the onion. Cut half into columns, half into cubes
Wash the fish, dab dry and cut into pieces. Bring bay leaf, lemon juice, stock, wine, 300 ml water and onion wedges to the boil. Season with salt. Put the fish into the stock and cook in boiling water for about 10 minutes. Melt 2 tablespoons of fat in a pan. Steam carrot strips for 4-5 minutes until al dente, season with salt, pepper and sugar. Wash parsley and shake dry. Pluck the leaves from the stems and chop them, except for a little bit for garnishing.
Measure out 100 ml of the fish stock. Melt 1 tablespoon of fat. Fry the onion cubes for about 2 minutes until transparent. Stir in flour, sauté, then deglaze with cream. Add stock, season with salt, pepper and sugar. Simmer the sauce for 3-4 minutes while stirring
Drain the potatoes, put them back into the pot with 2 tablespoons of butter and chopped parsley, toss through. Remove fish from the stock. Arrange fish, carrots, potatoes and sauce, garnish with parsley. Sprinkle sauce with coarse pepper. Add lemon wedges