Heat oil and butter in a large soup pot over medium-low heat.

Add celery, onion, sage and parsnips.

Cook, stirring occasionally until onions are translucent, about 5 minutes.

Add sherry, stir well, and bring to a boil.

Reduce heat to medium-low and simmer until liquid is reduced by half, about 10 minutes.

Add broth, salt and pepper and bring to boil.

Reduce heat and simmer gently, uncovered, until parsnips are completely tender, 30 to 45 minutes.

Remove from heat and allow to cool slightly.

Puree soup in batches in a blender. Return soup to pot.

Whisk in half-and-half and reheat gently over low heat.

Taste and adjust seasoning before serving.