In small bowl mix oil, vinegar,garlic,honey, worcestershire,tarragon and mustard. Using a small knife remove peel and pits from grapefruit. Working over bowl of dressing to catch juices, cut between membranes to release segements. Place lettuce ( torn ) on large plate. Arrange grapefruit and avocado slices on lettuce. Drizzle witrh dressing to taste, sprinkle with walnuts and serve.