Cut the fish fillets into bite-sized pieces. In a bowl, combine fish sauce, lime juice, soy sauce, palm sugar, minced garlic, sliced chilies, and minced galangal or ginger. Mix well.
Stir in chopped cilantro, Thai basil, and finely chopped lemongrass. Add the fish pieces to the marinade and mix to coat well. Let it marinate for at least 30 minutes, or up to 2 hours for more flavor.
If using banana leaves, cut them into large squares and lightly steam or wilt them to make them pliable. If using aluminum foil, cut into squares.
Place a portion of the marinated fish mixture in the center of each banana leaf or aluminum foil square. Fold the edges over to create a sealed packet.
Place the wrapped packets in a steamer basket. Steam over boiling water for about 20-30 minutes, or until the fish is cooked through and tender.
Carefully unwrap the packets and transfer the Mok Pa to a serving dish.
Enjoy your meal!