Peel and wash the potatoes and cook them in salted water for about 20 minutes until soft. Peel and wash the carrot and celery and cut them first into thin slices, then thin strips. Clean, wash and cut the leek into thin strips.
Simmer crème légère and 2 tablespoons of stock in a saucepan for about 2 minutes. Wash the fish, pat dry and season with salt, pepper and a little lemon juice. Melt the margarine in a shallow wide pot.
Briefly sauté the vegetable strips, season with salt and deglaze with 1 tablespoon of stock. Place the fish on top and stew covered at medium heat for 3-4 minutes. Wash the chervil, shake dry and chop finely, except for one leaf for garnishing.
Add chervil to the sauce and season with salt, pepper and a little lemon juice. Bind with sauce thickener. Arrange the fish on the vegetables and pour the sauce over them. Garnish with chervil. Add potatoes.