1. Place all dressing ingredients in the bowl of a food processor. Blend until smooth and creamy. If necessary, adjust salt and pepper to taste.
2. To assemble salad, pour dressing over broccoli and toss to combine evenly (this can be done a day in advance).
3. Arrange almonds around the edge of the serving bowl, and sprinkle the grapes (and onion, if using) in the middle.