Put all dressing ingredients in a medium size mason jar. Shake vigorously to blend. Taste and season with salt and pepper. Set aside.
Using sharp knife, cut between membranes of grapefruits to get just the segments.
Place the arugula (or spinach or favorite salad mix) in a large salad bowl, and place fennel slices on top.
Arrange grapefruit segments and avocado slices on top of the fennel.
Give the dressing a good shake and then drizzle over salad. Garnish with toasted walnuts. Serve immediately.