Dipping sauce: Combine lime juice, sugar, fish sauce and chillies with 3 tablespoons water.
Blanch asparagus until tender crisp, but still bright green. Cut spears in half across bunch.
Rolls: Half fill a large bowl with lukewarm water. Dip one whole sheet of rice paper in water and lay flat on a plate.
Place 2 prawn halves in a line 3cm from top of wrapper. Top with a little shredded lettuce, 2 Thai basil leaves, 1 slice avocado, cucumber strips, and 2 pieces of asparagus.
Fold sides of rice paper over filling, and then roll firmly from bottom to top, or how you prefer.
Place each roll on a serving plate and cover with a slightly dampened cloth, while preparing remaining rolls.