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 becomes translucent, about 5 minutes. Add 3 whole habanero chiles and 3 whole Fresno chiles and cook, stirring occasionally, until softened, about 5 minutes.
Step 2Pour in the wine, increase the heat to high and bring to a boil, stirring and scraping the saucepan. Continue boiling until the wine has reduced by half. Stir in the calamansi juice, then reduce the heat to low.
Step 3When the liquid has stopped bubbling but is still steaming, add the butter one cube at a time while stirring, letting each cube melt before you add the next. You want a velvety texture, but you don’t want the sauce to break. Once all the butter has been added and the sauce looks silky, stir in the fish sauce. Taste the sauce and add more fish sauce or salt to taste. Keep warm over low heat, stirring occasionally.
Step 4Prepare a charcoal grill for direct, medium-high-heat grilling or heat a gas grill to medium-high.
Step 5Lay 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 calamansi rinds and remaining habanero and Fresno chiles. Wrap the fish tightly in the banana leaf, leaving the tail exposed, then tie with kitchen twine along the length of the fish to keep the leaf packet shut.
Step 6Put 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 7Transfer 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.