Preheat the oven to 180°C (160° fan-forced). Line a roasting pan with foil.
In a medium bowl combine oil, coriander, mint, fish sauce, lime zest and juice and sugar. Using a small, sharp knife make incisions over top of lamb. Place lamb on lightly oiled metal rack set over prepared roasting pan, spoon over half the marinade and season lamb. Sprinkle over lemongrass and chilli. Cover with foil and roast in oven for 1 hour.
Remove foil from lamb and cook for a further 40-45 minutes, or until lamb is cooked to your liking. Remove lamb from rack and set aside on a tray loosely covered in foil for 15 minutes to rest.
Meanwhile, prepare noodles according to packet instructions. Rinse noodles with cold water and drain well. Place noodles in a large bowl and add carrot, cucumber, capsicum and remaining marinade.
Slice lamb and serve with noodle salad, butter lettuce leaves, coriander and mint leaves, fried shallots and lime wedges.
For a full leg of lamb double the ingredients for this recipe.
This marinade would also work well on lamb cutlets, loin chops or leg steaks.
Use leftover lamb to make Banh Mi for lunch the next day – grab some crusty baguettes, spread with mayonnaise and fill with gently warmed, sliced lamb, salad and herbs.