Banana leaves are perfect for wrapping a whole fish and insulating all of the aromatic flavors of this dish during grilling. A quick green sauce loaded with herbs, ginger and fresh green chilies provides a cool and spicy kick for the meaty grilled fish.
Make the sauce: Combine the cilantro, mint, chiles, ginger, scallion, coconut, sugar and 1 teaspoon salt in a blender and pulse to make a coarse paste. Add the coconut milk, lime juice, vegetable oil and fish sauce and blend to make a smooth, slightly thick sauce. Season with salt. Transfer to a bowl and refrigerate until ready to serve.
Make the fish: Preheat a grill to medium high. Lay the banana leaves on a work surface, overlapping each about halfway, so the leaves are large enough to wrap around the fish, with the tail and head sticking out slightly. Lightly brush the banana leaves with vegetable oil. Cut three 24-inch pieces of kitchen twine and run under water to moisten (this will prevent the twine from burning on the grill).
Make 2 or 3 slits crosswise on each side of the fish, cutting about 1/2 inch into the flesh. Season the fish inside and out with 1 1/2 teaspoons salt total. Stuff the lime and ginger slices, chiles, and cilantro and mint sprigs into the cavity of the fish. Set the fish at one short end of the banana leaves and roll it up, leaving the head and tail sticking out, then tie together in 3 places with the kitchen twine.
Once the grill registers 425˚ F to 450˚ F, grill the fish until the banana leaves are charred, about 12 minutes. Flip and grill until the fish is cooked through, 12 to 14 more minutes. (Because the fish is wrapped, it is difficult to test for doneness. The juices will run clear from the head and gills when poked.) Let the fish rest 5 minutes, then untie and remove the banana leaves. Lift the flesh from the bones and serve with the cilantro-mint sauce.
Overlap the banana leaves to create one large surface for the fish. Once you wrap the fish, tie it with damp twine — dry twine can burn on the grill.