Start a wood or charcoal fire or preheat a gas grill (or broiler); the fire should be quite hot and the rack about 4 inches from the heat source. While it’s heating, make the mayonnaise and combine it with all the remaining ingredients except the fish. (If you’re making the mayonnaise in a food processor or blender, you can save yourself some mincing by adding the garlic at the beginning and pulsing in the other solid ingredients while the sauce is still in the machine.)
Grill the fish, turning when nicely browned, about 4 or 5 minutes, then cook for 4 or 5 minutes more. The fish is done when just a bit of translucence remains in its center; use a thin-bladed knife to check. Serve hot or cold, with the rémoulade sauce.
