2 large pink grapefruits, peel and white pith removed
1 pound fennel bulbs, trimmed, cut into paper-thin slices
Whisk first 10 ingredients in large bowl to blend. Season with salt and pepper.
Using sharp knife, cut between membranes of grapefruits to release segments.
Spread fennel slices over large platter. Arrange grapefruit segments and avocado slices atop fennel. Drizzle dressing over salad. Arrange arugula atop salad.