Preheat a barbecue grill or chargrill pan on medium-high. Cook the pineapple for 2-3 minutes each side or until charred.
Place all the dressing ingredients in a food processor and process until well combined and almost smooth. Season.
Arrange the lettuce on a serving platter. Top with the pineapple, prawns, avocado and onion. Drizzle over the dressing to serve.
