Melt the butter in a medium saucepan over medium heat. Add the onion and cook until translucent. Drain the celery root and add it and the potato to the onions. Pour in enough chicken stock to cover. Heat to boiling, then reduce the heat and simmer until the celery root is very tender. Add the fennel and simmer 5 minutes more.
Puree the soup in a blender or processor and thin if necessar