Sorrel cream soup (Potage à l'oseille)

1. In a pot, soften the white part of the sliced leek in butter until translucent.

2. Add the diced potato, pour in the vegetable stock, season with salt and pepper and cook for about 15 minutes until the potato is tender.

3. Drop in the roughly chopped sorrel, which wilts in a few minutes (turning from bright to olive green).

4. Blend the soup smooth with a stick blender.

5. Mix the egg yolks with the cream and, tempering with the hot (but not boiling) soup, stir them in in a stream, stirring constantly so it thickens silkily (do not let it boil again).

6. Serve at once, adjusting the salt to taste.

Tip: Sorrel is distinctively, pleasantly tart – it gives the soup its freshness. Never boil the egg-yolk-and-cream liaison or it will split; it is what makes the soup velvety.