Open cod, drain any excess liquid, and pat dry with paper towels. Cut into six portions. Season lightly with salt.
Combine lemongrass marinade ingredients in a blender until smooth. Pour marinade over the fish and make sure all parts are covered with the marinade. Refrigerate for about an hour.
Heat your grill to medium heat. Preferably charcoal fire or wood grill if you have them available.
Lightly heat each piece of banana leaf over grill to make them pliable to fold to prevent ripping. (If using foil paper, skip to step 4).
Place 1 tablespoon of marinade in the center of each banana leaf, followed by 1 piece of fish. Fold banana leaf/foil paper to enclose the fish inside. Use the skewer to keep the banana leaf closed. Repeat with all portions of fish.
Place each banana leaf/foil paper-wrapped portion on the grill. Cook for 8-10 min on each side until the fish is steamed and cooked through.
While fish is resting, make the Nam Jim Jaew (Thai chili dipping sauce). Combine ingredients in a bowl and mix well.
Open each banana leaf/foil paper, add limes and garnish.
Serve over rice with dipping sauce.