The fish. If you’re using the whole fish, butterfly it: Flip the fish over so that the visceral cavity is on top. Using a pair of kitchen shears, firmly (but gently, so as not to tear the flesh) snip the rib bones (which frame the cavity) free from the back bone (which runs down the center of what is from this perspective the bottom of the cavity). Then, starting at the end nearest the head, use a thin sharp knife to cut along one side of the backbone and the bones that protrude down from it, until you reach what would be the top of the fish as it swims. Don’t cut through. Continue cutting along the spine and bones toward the tail, always angling your knife toward the bones, until you reach the end of the cavity. Now, on the same side of the spine you’ve been cutting, cut all the way down to the tail. Cut over the little hump of the backbone, then continue down until you reach what would be the top of the fish as it swims. Again, don’t cut through. Half of the fish is now free. Do the same on the other side. Holding the (mostly free) backbone firmly, break or cut it from the head and from the tail; discard or save for stock. With a cleaver or heavy knife, split the head without cutting all the way through it. You now have a completely butterflied whole fish with only the lower rib bones attached (plus a few pin bones in the heart of the fillets). Feel free to cut or pull them out or leave them in. Whether you’re using the whole fish or fish fillets, lay them on a rimmed baking sheet.
The marinade. In a small bowl, break up the achiote paste and, with the back of a spoon, work in the lime juice and a generous ½ teaspoon salt. Use the spoon to smear the marinade over the the flesh side of the fish. (You may have a little left over; use it on some roasted vegetables before you roast them.) Cover and refrigerate while you’re setting up the grill.
Prepare the grill. Twenty to 30 minutes before cooking, heat a gas grill to medium-high or prepare a charcoal fire, letting the coals burn until they are covered with a gray ash and about medium hot. For grill-roasting the fish, turn the burners to low under half of the gas grill or bank the coals to one side of the grill.
Grilling in a wire basket. Brush or spray oil on both sides of a hinged grill basket (I like the one that has flexible wires that will mold around the fish). Spray or brush oil on the flesh side of the fish fillets or butterflied whole fish. Lay the banana leaf on one side of the basket, then lay in the fish, skin-side down. Close the basket, then flip it over onto the hot grill so that the banana leaf is on top. Cook until the fish begins to brown, 4 or 5 minutes (it may be a little longer for the whole fish), depending on the heat of your grill. Flip the basket and cook until the fish flakes under firm pressure, anywhere from 3 to 7 minutes, depending on the thickness of the fish and temperature of the grill. Carefully open up the basket, freeing any sticky bits of fish. For the whole fish, break off any remnants of burn banana leaf, then slide it onto a serving platter still on the leaf (I like to lay a fresh banana leaf first, just for a nice-looking presentation. For the fillets, I slide leaf and fish onto a cutting board. Sprinkle the fish with salt (a beautiful finishing salt is welcome here).
Grill-roasting the fish. Spray or brush oil on both sides of the fish fillets or butterflied whole fish, then sprinkle with salt. Lay the banana leaf onto the cooler side of your grill and immediately to with the fish, skin-side down. Cover the grill and cook until the fish flakes under firm pressure, usually 7 to 12 minutes, depending on the thickness of the fish and the temperature of the grill. Slide 2 large spatulas or a pizza peel under the fish and transfer the whole fish directly to a serving platter, the fish fillets to a cutting board. Sprinkle the fish with salt (a beautiful finishing salt is welcome here).
Serve. For the whole fish, I like to use a small spatula to free half of each fillet from the skin and transfer it to a warm dinner plate. For the fillets, cut the leaf into quarters (breaking off any burnt pieces) and use a spatula to transfer to each piece to a dinner plate. Serve with the lime wedges, salsa, pickled red onions (if you have them) and warm tortillas for making some of the best tacos ever.