Saute onions and celery in the butter until tender over medium heat.
Add herbs and cook 2 more minutes. Add flour to make a rue (paste) and cook until thick. Add warm water and lemon juice and bring to a simmer, stirring to keep smooth. Add ½ and ½ and milk and bring to a simmer, stirring. Add fish and bring back to 160 degrees. Add potatoes and simmer 10 more minutes. Garnish with dill sprig.
