Cut the prawns into small pieces and the pound the prawns to a paste using a mortar and pestle.
In a large mixing bowl combine the pork and crab and then season with salt, pepper, sugar and fish sauce. Mix.
Add noodles, mushroom and onion. Knead consistently for 10 minutes or until your arms gets tired.
One at a time quickly dip one rice paper wrap into a pan of warm water and place it on a chopping board or bench.
Take one heaped tablespoon of the filling and place it on the bottom edge of the rice paper.
Fold the two adjacent sides, one on top of the other into the centre. Roll the apex to form a nice firm roll.
Repeat until you have filled all the rice papers. All rolls should be 10cm x 3cm in diameter.
Heat oil in a wok to 180 degrees and fry the parcels in 3 separate batches for 6 - 8 minutes or until lightly browned and crisp (they will rise to the top when they are ready).
To serve place some of the herbs, bean sprouts and a spring roll into a lettuce leaf, dip into nuoc mam cham sauce.
Dipping Fish Sauce - nouc mam cham:
Combine the fish sauce, rice vinegar, water and sugar in a saucepan and place over medium heat. Stir well and cook until just before boiling point is reached, then allow to cool. To serve, add garlic and chilli then stir through with lime juice.