1.Place ingredients for lime and sweet chilli dressing in screw-top jar; shake well.
2.Score fish both sides through thickest part of flesh; place on large tray, drizzle with half of the dressing. Cover; refrigerate 1 hour.
3.Meanwhile, trim banana leaf into four 30cm squares. Using tongs, dip one square at a time into large saucepan of boiling water; remove immediately. Rinse under cold water; pat dry with absorbent paper.
4.Place leaves on work surface. In a medium bowl, combine chilli, lime leaves, onion, coriander and basil. Halve lemongrass sticks lengthways, then halve crossways to make 8 pieces.
5.Place two pieces of lemongrass on each leaf; place one fish on each. Top fish with equal amounts of the herb mixture. Fold opposite corners of the leaf to enclose centre part of fish; secure each parcel with kitchen string.
6.Place two parcels, in single layer, in large bamboo steamer; steam, covered, in two batches, over wok of simmering water about 15 minutes or until fish is cooked through. Unwrap fish; drizzle with remaining dressing.