Peel, trim and coarsely chop parsnips.
Saute the parsnips in butter with shallots and salt for about 10 minutes.
Roughly chop pears and continue to cook over medium heat, adding a bit more butter, a generous squeeze of lemon juice and a pinch of ground coriander.
Add this mixture to chicken stock and white wine.
Bring it up to a boil, then reduce the heat to a slow simmer and cook until parsnips are soft.
Remove from the heat and puree with a hand-held blender, being careful of the hot liquid. (A regular blender can be used, of course, but puree in batches.)
Check for seasoning, especially salt, and add in heavy cream.
Continue on low heat for another 10 minutes.
Swirl a bit of crème fraiche on the top of the ladled soup and garnish with a sprig of chervil to serve.