1. Combine first 4 ingredients. Gradually add oil, stirring with a whisk.
2. Combine 1 tablespoon dressing, fennel, and celery in a bowl; cover and refrigerate 2 hours. Set aside remaining dressing.
3. Arrange lettuce on a platter; top with celery mixture, 2 cups grapefruit sections, 1 cup pomegranate seeds, avocado, and pine nuts. Drizzle with remaining dressing.