Heat a soup pot over medium heat and melt the butter. When it begins to foam, add the celery, onions and garlic; season with celery salt and pepper. Add the fresh bay leaf to the pot, partially cover and cook until very tender and soft, about 20 minutes, stirring occasionally.
Add the potatoes, celeriac (celery root) and stock or poaching liquid to the pot and bring to a boil. Reduce the heat to a low rolling boil and simmer, partially covered, for another 20 minutes. Stir in the cream and purée with an immersion blender. Taste to adjust the seasoning and serve in shallow bowls.
For a make-ahead meal, cool the soup, then store. To re-heat, place the soup in a soup pot, bring to simmer and taste to adjust the seasoning. Serve in shallow bowls.