Set a 4-quart saucepan over medium-high heat. Add the bacon and cook, stirring frequently, until crispy and fat is rendered, about 4 minutes. Remove the bacon using a slotted spoon and transfer to paper towels to drain. Remove 1 tablespoon of the rendered bacon fat and set aside for another purpose. Add the leek, onions, parsnips and celery and cook, stirring occasionally, until softened, 4 to 5 minutes. Add the garlic to the pan and saute until fragrant, 45 to 60 seconds. Add the celery root, potato, stock, salt, pepper, and rosemary to the saucepan and bring to a boil; reduce heat to a simmer and cook until the celery root is very tender, 45 minutes to 1 hour.

When the celery root is tender, remove the saucepan from the heat and puree the soup using an immersion blender (or in batches using a bar blender), until smooth. Stir in the heavy cream and cider vinegar. Finely chop the reserved crispy bacon. Taste, and re-season the soup if necessary and serve, garnished with the crispy bacon and snipped chives