1.Combine chopped fresh coriander, coriander root, lemon grass, garlic, juice, rind, chilli, sugar and sauce in large bowl; add lamb, turn to coat in marinade. Cover; refrigerate 3 hours or overnight.
2.Preheat oven to moderate (180ºC/160ºC fan-forced).
3.Strain lamb through fine sieve over medium bowl; reserve marinade solids in strainer and marinade in separate bowl for thai salad. Cut each lamb roast in half horizontally.
4.Trim banana leaves 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. Banana leaf squares should be soft and pliable.
5.Using fingers, press lamb halves all over with marinade solids; place each half in the centre of one of the banana leaf squares. Fold banana leaf over lamb to enclose; secure each parcel with kitchen string.
6.Place lamb parcels, in single layer, in large shallow baking dish; spray parcels with cooking-oil spray. Cook, uncovered, in moderate oven about 30 minutes or until roast is cooked as desired.
7.Meanwhile, make Thai salad. Combine ingredients in large bowl. Place remaining reserved marinade in small saucepan; bring to a boil. Reduce heat; simmer, uncovered, 2 minutes. Cool marinade 10 minutes, pour over salad; toss gently.
8.Serve lamb on opened banana leaf with salad.
Banana leaves can be ordered from greengrocers.Note