Fry the onion in butter until translucent. Add flour and make a light roux with milk and water and cook for at least 20 minutes.
Add the mustard, cream, salt, pepper and pepper berries. Season to taste with a little lemon juice.
Clean the fish and season with herb salt, pepper and a little lemon zest. Fry briefly on one side in the hot clarified butter, turn and finish cooking on the switched off stove.
Boiled potatoes or rice and a fresh salad go very well with it.