Put the lemon grass, spring onion and garlic in a food processor and whizz to a paste. Add the prawns and herbs and whizz again to blend into the paste. Add the pork, sugar and fish sauce and whizz once more.
Tip the mixture onto a chopping board and knead lightly by hand to make sure the ingredients are well mixed. Bring together into a ball. Divide this into quarters and then divide each quarter into eight so you have 32 pieces in total.
Roll each piece into a small ball and place on a plate in a single layer. Cover loosely with cling film and put in the fridge for at least 30 minutes to firm up.
Meanwhile make the dipping sauce by stirring all the ingredients together. Place in small serving bowls.
Heat the oil in a frying pan. When hot fry the meatballs for about 6 minutes until cooked through and browned all over. Serve with the dipping sauce.