Whisk all vinaigrette ingredients together EXCEPT olive oil. While whisking briskly, slowly drizzle in olive oil. Set aside
In a small stockpot over medium heat, melt butter then sautée onions and garlic until fragrant. Add in couscous and cook until couscous starts to brown, about a minute.
Add in chicken stock, then cover and reduce heat slightly. Cook for 10 minutes or until liquid is absorbed. Test couscous to ensure properly cooked. Fluff with a fork and allow to cool.
Drizzle in vinaigrette, then toss in cucumber, walnuts, cranberries, red bell peppers and cilantro. May be served immediately or refrigerated for later service.