Place the avocado, mango, garlic, ginger, vinegar and oil in a large bowl. Season with salt and pepper. Add the lime juice and gently stir to combine. Set aside for 5 mins to develop the flavours.
Meanwhile, divide the lettuce among serving plates. Drizzle with the extra vinegar and extra oil. Season with salt. Drizzle with mayonnaise.
Divide the prawns among serving plates and top with the avocado mixture. Sprinkle with coriander to serve.
