Whisk the soy sauce, sake and mirin and the sesame oil together in a bowl and set this dressing aside.
Place each banana leaf into a bowl so that it follows the shape of the bowl. Place the carrots, spring onions, basil, thyme leaves and ginger into a clean bowl and mix well. Place a little of the vegetable, herb and ginger mixture into the dip of each banana leaf (reserving half of the vegetables for later), then top with a piece of brill.
Spoon a little of the dressing over the top, then cover with the reserved vegetable, herb and ginger mixture and any remaining dressing.
Fold the banana leaf over to cover the filling and tie with cook's string to completely seal as a parcel.
Place the banana leaf parcels onto a baking tray and transfer to the oven to cook for 10-12 minutes.
To serve, place each parcel onto a plate and open them at the table in front of your guests.