For the couscous: Heat the oil in a 2-qt. saucepan over medium heat. Add the onion and celery and cook for 2-3 minutes, stirring occasionally, until the vegetables are softened.
Stir in the couscous. Cook and stir for 1 minute until coated with oil and lightly toasted. Add the water and bring to a boil, stirring. Remove from the heat and cover with lid. Let stand for 30 minutes, until cooled and the liquid is absorbed, uncovering occasionally to fluff with a fork.
In the meantime, make the dressing: In a bowl, whisk together the lemon juice, yogurt, oil, gingerroot, garlic, cumin, ground coriander and pepper.
To make the salad: Transfer the couscous to a large serving bowl. Add the cranberries (or raisins), chickpeas, bell peppers, cilantro (or parsley) and green onions. Add the dressing.
Toss all the ingredients together. Garnish with the lemon wedges, if using.