To prepare salad, combine first 4 ingredients in a large bowl. Pour 2 cups boiling water over the couscous mixture; cover and let stand 5 minutes. Fluff with a fork; cool. Stir in onions, basil, and chickpeas.
To prepare dressing, combine lemon juice and the remaining ingredients in a jar; cover tightly, and shake vigorously. Pour over couscous mixture, and toss well to combine. Cover and chill for 1 hour.