Step 1Heat the olive oil in a large saucepan over medium-high heat. Add the onion and garlic, season with a pinch of salt and cook, stirring occasionally, until the onion is translucent, about 5 minutes. Add 3 whole habanero chiles and 3 whole Fresno chiles and cook, stirring occasionally, until softened, about 5 minutes. Stir in the bay leaves.
Step 2Pour in the vinegar, stir to combine and bring to a boil. After the strong smell of vinegar burns off, about 2 minutes, stir in the coconut milk and soy sauce. Bring to a boil, reduce the heat to maintain a steady simmer and cook, stirring and scraping the saucepan occasionally, until the liquid is thick enough to coat the back of a spoon, about 15 minutes. Stir in 1 teaspoon black pepper, then taste and season to taste with salt. Keep warm over low heat.
Step 3Prepare a charcoal grill for direct, medium-high-heat grilling or heat a gas grill to medium-high.
Step 4Lay a banana leaf on a work surface. Using kitchen shears, trim any hard veins off the edges and discard. Use the same shears to cut the fins and interior gills off the fish and discard. Arrange the fish at one end of the banana leaf. Sprinkle the fish inside and out with salt, then stuff its cavity with the remaining habanero and Fresno chiles. Wrap the fish tightly in the banana leaf, leaving the tail exposed, then tie shut with kitchen twine.
Step 5Put the fish on the grill, cover, and cook, flipping once halfway through, until opaque throughout, about 20 minutes. You can check for doneness by inserting a paring knife into the exposed tail meat. The knife should slide through easily, and the meat should be white.
Step 6Transfer the fish to a platter. Snip the twine and unwrap the fish, leaving it atop the banana leaf on the platter. Spoon the sauce all over the fish and serve immediately with any extra sauce in a bowl on the side. Serve immediately.