Using a small, sharp knife, remove peel and white pith of grapefruit. Working over a large bowl to catch the juice, cut between grapefruit membranes, slicing out segments. Place lettuce on a large plate. Arrange grapefruit sections and avocado atop greens. Drizzle salad with dressing, sprinkle with walnuts, and serve.
Whisk ingredients together until well blended.