Whisk together ingredients for the dressing; chill. Toss gently the scallions, red and green pepper slices and avocado slices (sprinkled with lemon juice to keep from turning brown); chill. When ready to serve: mix together chopped lettuce and watercress; spoon onto 4 individual plates. Top with scallions-pepper-avocado mixture and then the crabmeat. Drizzle with dressing.