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I've traveled down to Florida's Gulf Coast nearly every year of my life to visit family, and if there's one constant beyond swimming pools, beaches, and Spanish moss, it's a good fish sandwich. The fish is usually lean, white-fleshed grouper, and you can choose to have it battered and fried, grilled, broiled, or sautéed. Sometimes it's blackened, a Cajun technique that involves rubbing the fish with a spice and herb mixture that turns dark when cooked. The sandwich is otherwise very simple. The bread is usually soft and tender and nondescript; the fixings no more than basics, like lettuce and tomato; the sauce mayo or tartar or remoulade.
It may sound like I'm underselling the sandwich here, pulling up adjectives like "simple" and "nondescript" and "basic," but I mean quite the opposite. A good fish sandwich should be all about the fish, uncomplicated in the best way—the kind of thing you want to eat as you wiggle your bare toes in warm sand and the sun sets over the sea. Nothing about this sandwich overshadows that moment. Not crusty bread that's difficult to bite through, not unconventional condiments, and not some fattier or more distinct fish, like tuna or salmon.
Truth is, a sandwich like this is pretty damned good up here in New York City, too, if only because, for a few short minutes, it allows me to escape the traffic and concrete. I can almost smell the salty sea breeze.
For this version, I decided to blacken the fish and grill it, though you could just as easily cook it under the broiler or in a cast iron skillet. I couldn't find grouper, so I picked up some mahimahi, another white-fleshed fish common to Gulf waters that makes an acceptable stand-in. Any kind of sea bass would also work well. To make the spice mixture, I combined some paprika, onion and garlic powder, dried oregano and thyme, freshly ground black pepper, and a pinch of cayenne for mild heat.
Grilling fish fillets isn't typically the easiest of things to do, since the delicate flesh can stick and break. But with just a little good technique, it's not at all hard to pull off. The spice rub helps, too, since it acts as a barrier between the fish flesh and the grill grate, reducing the chances of sticking.
The most important thing is to preheat your grill well, then clean and oil the grill grate. The hotter and cleaner the grill grate, the more quickly it will sear the fish and prevent it from bonding with the metal. Whatever you do, don't try to move the fish around until it's had a minute or two of direct heat, which should be enough time to allow the fish to release from the grill if it got stuck initially. A thin slotted fish spatula is ideal for this kind of delicate work.
I prefer a two-zone indirect fire for this kind of thing, since I can start the fish on the hot side, getting the exterior set and lightly blackened, and then move it to the cooler side to finish cooking, if it needs more time. Note, though, that "blackened" does not in any way mean burned. The spice rub will darken on the fish, but you should not leave it over direct heat so long that it chars to the point of being acrid and bitter.
There are a few good ways to judge doneness on a piece of fish like this. The first is to use an instant-read thermometer. When the fish registers 140°F (60°C) in its center, it's done. This method works well with thicker pieces of fish, in which you can't easily see into the center.
Another good trick is to push a thin cake tester into the fish; if you feel resistance as you push, the fish isn't ready yet. Once it can slide through easily, you should be good to go. (This is because the fish membranes that provide resistance break down right around doneness temps.) Just make sure you don't reuse the same hole each time you test.
The last method is to use any natural seams in the fish, such as the division on a fillet where the spine was attached, to peer into the center. When only the faintest trace of translucence remains at the very center, the fish will be done—once it's off the heat, carryover cooking will take care of that last trace.
With that, your fish, and therefore your sandwiches, are ready. Toast the buns, build the sandwiches, and serve. If you did, hypothetically, decide to slip a couple crispy slices of bacon in there, I don't think Poseidon himself would object. Simplicity is good, but simplicity plus bacon is hard to argue with.