Preheat oven to 180 degrees C - using the middle shelf.
Flake fish and remove bones add parsley to fish.
Slice top off French stick lengthways leaving enough bread to for a hinge between the top and bottom of the loaf. Hollow out the bottom section leaving about 1/2inch/1cm of thickness. Brush with melted butter and spread the mustard on the bottom section only.
Saute the onion and celery till onion is golden. Add to fish and parsley. Add the mayonnaise, lemon juice, and sauce.
Fill the cavity of the loaf with this mixture and put lid back on top
Bake in middle of oven for 15 mins. at 180c.
Use margarine and a dairy free mayo for a dairy-free meal.