Combine marinade ingredients in a glass or ceramic baking dish. Whisk to combine. Using a sharp knife, score several diagonal lines along the length of the fish, down to spine, on either side. Marinate 1 hour in the refrigerator. After marinating, preheat grill or broiler. Meanwhile, prepare salsa: Preheat the broiler. Place the tomatoes, garlic, chilies and onion on a baking tray. Tuck the garlic underneath the vegetables to avoid blackening.
Broil, turning frequently, until well charred, 15 minutes. Set aside to cool. Transfer roasted ingredients to a food processor fitted with the metal blade or blender and puree until smooth. Season with salt and pepper. Whisk in the olive oil and add the avocado. Set aside. Blanch greens in a large pot of boiling, salted water, just until the water returns to a boil. Refresh with iced water and drain. On a large baking sheet, arrange greens, overlapping to form 2 rectangles as long as each fish and more than twice as wide. Place each fish in the middle of each rectangle and wrap with greens so that only the head and tail are exposed. The dampness will make them cling.
Transfer fish to grill, reserving marinade. Cook 7 to 8 minutes per side, until greens, head and tail blacken. It will take 2 spatulas, one at the head and one at the tail, to turn fish. Don't be concerned about greens sticking. To test for doneness, try to pull out a dorsal fin from the top of the fish. If it slides out easily, the fish is done. To serve, place whole fish on a serving platter. Using a dull knife and soup spoon, scrape off any remaining skin and remove fins. Cut along spine first and serve the top fillet. Lift tail, insert knife between bones and flesh, and run it along length. All (or most) bones should easily lift out. Repeat for second fish. Serve with the salsa.
c.1997, M.S. Milliken & S. Feniger, all rights reserved