Method – Using a small sharp knife or scissors (my preferred method), carefully slice open each prawn shell along the back and pull out the black vein. Try to keep the rest of the shell intact. Place the deveined prawns in a shallow dish and set aside. Put the fish sauce in a small bowl with the sugar, and beat together until the sugar has dissolved completely. Add the oil and lemongrass, then mix well. Pour the marinade over the prawns, using your fingers to rub it all over the prawns and inside the shells too. Cover the dish with cling wrap and chill for at least 4 hours. Cook the prawns on a bbq or grill for approx. 2-3 minutes (or to your own liking) each side. Serve on a bed of vermicelli noodles, lettuce and mint leaves. Drizzle with spring onion oil, Vietnamese dipping sauce and crushed peanuts.
Recipe from: http://www.opensourcefood.com/people/CertifiedEater/recipes/grilled-lemongrass-prawns
Changes were made. http://creativecommons.org/licenses/by-sa/3.0/