In a large bowl, mix dressing ingredients until blended. Season with salt, then set aside.
To serve, halve and stone the avocados. Dice the flesh while it’s still in the skin then scoop into a bowl. Add the prawns and toss in the dressing. Line six small serving bowls or tumblers with two or three chicory leaves. Pile in avocado and prawn mixture. Top with watercress and lime wedges. Serve with prawn crackers.