Frozen banana leaves, which are inedible, are available at Asian markets. If you can't find them, parchment works just as well. Recipe adapted from Amanyara Resort, in Turks and Caicos.
Toast curry powder in a dry skillet over medium heat, swirling constantly, until very fragrant, about 1 minute. Let cool.
Puree garlic, chopped scallion, ginger, fish sauce, agave, coconut milk, lime juice, and 3/4 teaspoon toasted curry powder in a small food processor until smooth.
Cut out four 10-by-12-inch rectangles from banana leaves or parchment; wipe leaves with a damp towel. With 1 long edge of each rectangle facing you, lay a fillet lengthwise in center of each, about 1 1/2 inches from bottom. Season each with salt and remaining toasted curry powder, and top with 1 tablespoon curry sauce. Fold short sides of rectangle over fish, then fold up bottom edge. Fold top edge over packet, and tuck underneath. (Don't worry if leaves tear a little.)
Place 2 packets in each layer of a 2-layer bamboo steamer or all 4 in a stainless steel steamer. Fill a large pot with 1 to 2 inches water, and place steamer in pot. Bring water to a boil, then reduce heat to medium. Cover, and steam until fish is just cooked through, 10 to 11 minutes, rotating layers halfway through if using bamboo steamer. Remove from steamer, unwrap, and blot excess liquid from the leaves or parchment with paper towels. Drizzle fish with remaining sauce. Top with tomatoes, sliced scallions, and cilantro, and serve each on a banana leaf or parchment with a lime wedge.
Healthy TrickTop fish with sauce before steaming it in a banana leaf or parchment to deeply infuse it with flavor.