Skinless and boneless filet, on oiled aluminum foil. Mix mayonaise with a spoon or two of prepared pesto sauce, slather it on the filet and bake in preheated 450F oven for 10 minutes. Or for less time if the fish is less than an inch thick.
If the filet is quite thin at the tail end, fold it under until thickness is more uniform, then cover with mayo.
For variation, instead of pesto:
Mix in sundried tomatoes packed in oil, with some italian seasoning, or
A bit of curry paste, or
Chopped capers and black olives, or
Any other combo you like.
If you want more of a crust, use the broiler and cook for 7 minutes instead of 10.