Heat a griddle pan and toast a tortilla for 30–60 seconds on each side, until lightly charred. Immediately push it into a small, deep bowl and set aside. Repeat with the remaining tortillas to make 4 bowl-shaped tortillas. Place one-quarter of the shredded lettuce inside each one.
Meanwhile, toss together the prawns, avocado, coriander and lime juice, and season to taste. Divide between the tortillas and serve with the lime wedges.