In a saucepan, add the stock, butter and couscous; bring to a boil, remove from heat stir in garlic and green onion, cover and let stand for 5 minutes. Remove garlic and fluff with a fork. Set aside to cool.
In a small bowl add vinegar, mustard and shallot, whisk together. Slowly whisk in the oil in a thin stream to emulsify. Add seasonings to taste. Can be refrigerated for 3 weeks, bring to room temperature before using.
In a large bowl, add all chopped ingredients and toss with a little of the dressing. Let stand for 5 minutes. Toss couscous and vegetables together adding a little more dressing as necessary, to taste.