Preheat the grill to medium-high heat. Soak the green lemongrass leaves in cold water and set aside.
Mix together the diced lemongrass, soy sauce, garlic, spring onion, red chile and ginger in a bowl.
Lay the fish on a chopping board. Create a fillet by using a sharp filleting knife to slice from the spine of the fish down to the belly without cutting the fillet off. Just leave it lying open.
Now add the lemongrass mixture to the inside of the fish and close the fillet back up.
Secure the fish by tying it up with the lemongrass leaves. Brush the fish with the vegetable oil, and then char grill the fish, 10 minute per side. Cut the lemongrass leaves and serve.