Melt the butter in a saucepan over a low heat. Add the shallots and celery and sweat, covered, for about 10 minutes. Add the stock, bring to the boil, then reduce the heat and simmer for about 10 minutes.
Separate out a small handful of the chervil and set aside. Roughly chop the rest, stalks and all, and add to the saucepan. Once the stock has returned to a simmer, cook for just 1–2 minutes, then remove from the heat. Transfer to a blender and purée until smooth. Return to a clean pan.
Whisk the egg yolks and cream together, then whisk into the soup. Reheat gently, stirring constantly, until just below a simmer; don’t let it get to simmering point. The egg yolk will thicken the soup slightly. Season with salt, pepper and a squeeze of lemon juice to taste.
Pick the leaves from the reserved chervil and chop finely, or leave whole if you prefer. Serve the soup in warmed bowls, finished with the extra chervil and a little swirl of cream.
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