How to Make It
In a blender or mini food processor, combine the palm sugar, lemongrass, shallots, garlic, ginger, chile and lime juice; pulse to make a slightly coarse puree. Transfer to a bowl and stir in the fish sauce and cilantro.
Light a grill. Score the skin side of each mackerel fillet 3 or 4 times, rub with oil and season with salt and pepper. Place a fillet skin side up in the center of each banana leaf. Fold up each leaf like an envelope to enclose the fish.
Grill the packets over moderately high heat until the fish is barely opaque in the center, about 3 minutes per side. Place a banana leaf packet on each plate and pass the sauce at the table.
The sauce can be refrigerated overnight. Bring to room temperature before serving.