Peel and wash the potatoes and cut them into cubes (approx. 1 x 1 cm). Peel and wash carrots and cut them into cubes. Heat butter in a pot. Sauté carrots and potatoes for 1-2 minutes, then deglaze with broth and cook covered for 8-10 minutes until soft. Remove the vegetables from the stock and keep warm. Fill up broth with cream to 300 ml and mix with mustard. Bring sauce to the boil and thicken with sauce thickener.
Season with salt, pepper and possibly some mustard powder. In the meantime wash the fish, cut the fillets in half and season with salt and pepper. Heat oil in a pan. First turn the fish in flour (knock off well), then fry in the pan for 3-4 minutes on both sides. Arrange vegetables, fish and sauce. Cut the cress from the bed and sprinkle over it. To garnish, sprinkle with mustard powder if necessary