Place the first five ingredients in medium bowl and whisk to combine. Cover and store in the fridge. Coat the avocado slices with the lemon juice. Arrange the salad greens on a large platter. Arrange the avocado, sliced grapefruit, prawns and pecans on top of the lettuce. Cover and store in the fridge until ready to serve. When ready to serve, warm the dressing to room temperature and whisk again. Drizzle the dressing over the salad and serve.