Melt the butter with the olive oil in a large pot or
Dutch oven over medium-high heat. Add the onion,
season with 1/2 teaspoon salt and cook, stirring
occasionally, until softened but not browned, about 4
minutes. Add the celery, celery root and 1 teaspoon
salt. Cook, stirring occasionally, until the celery
is tender, about 7 minutes.
Add the bay leaf, chicken broth and 2 cups water and
bring to a boil. Reduce the heat to medium low and
cook until the celery root is very soft, about 20
minutes. Discard the bay leaf. Add the heavy cream
and return to a simmer, then remove from the heat and
let cool slightly.
Working in batches, transfer the soup to a blender
and puree until smooth, adding some herbs to each
batch; return to the pot and reheat if needed. Stir
in the celery salt and serve with toppings.
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