When making fish for a crowd, I always go with a whole fillet rather than portions. It is by far the most foolproof method and looks pretty darn majestic sitting on the table surrounded by a big dish of tarragon-chive mayo and all the fixings. Slow-roasting the fish gives you a fillet that could never in a million years be described as dry. Just don’t expect the skin to be crispy.
Note: This dish is so, so good served cold; it’s the perfect make-ahead dinner.