For pickled onion, bring sugar, vinegar and water to the boil and pour over the sliced onion in a small bowl. Set aside for 30 minutes and then drain.
For guacamole, place avocado flesh into a medium bowl and roughly mash with a fork. Mix through spring onion, lime juice and season with salt and pepper.
Mix together the mayonnaise and chilli sauce in a small bowl.
Place chopped prawns into a medium bowl and mix through olive oil and salt and pepper.
Heat the tortillas either in a low oven, wrapped in foil, or the microwave covered in damp paper towel.
To assemble, place a heaped tablespoon of shredded lettuce onto the base of a tortilla. Top with prawns, a teaspoon of guacamole and a few pieces of pickled onion.
Arrange the tacos in a serving tray with bowls of extra lettuce, guacamole and pickled onion.
Drizzle the tacos with chilli mayonnaise, serve with lime wedges and garnish with coriander.
