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