How to Make It
Preheat grill to medium-high (about 450°F). Stir together cilantro, olive oil, shallot, lime juice, chile, lemongrass, brown sugar, ginger, and fish sauce in a medium bowl.
Sprinkle fillets with salt. Place banana leaves on grill grates; top each leaf with 2 flounder fillets. Spoon 1/4 cup shallot mixture evenly over fillets. Grill, covered, until flounder is opaque and cooked through, about 5 minutes.
Using 2 spatulas, carefully remove fish-topped banana leaves from grill. Place each fillet on a fresh banana leaf for serving, if desired; divide remaining shallot mixture among fillets.