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.
If you're using arugula, spinach or lettuce, put that in a salad bowl, add fennel and grapefruit.
Toss with some of the dressing.
Divide among 6 plates, arrange grapefruit segments and avocado slices on top of the salad on each plate.
Add an extra drizzle of dressing, and sprinkle toasted nuts on top.
Serve immediately.