Mix the oil, lime juice and 7-spice seasoning well. Coat the prawns with the mixture, cover and leave to marinate in the fridge for about an hour.
Combine the sugar with all the ingredients up to and including fish sauce in an aluminium cooking tray and mix well. Add the spring onions and mix together.
Grill the onions, covered, for approx. 15 mins. over a medium heat (approx. 220 °C.
Season the prawns with salt and place on the skewers. Lay the skewers on the hot grill and cook alongside the onions for approx. 2 mins. on each side.
Plate up the prawns and onions and serve with slices of lime.