In a medium saucepan, bring 2 cups water to boil. Stir in couscous and immediately remove from heat. Cover and let stand until water is absorbed, about 8 minutes. Fluff with a fork and transfer to a large bowl.
Stir in green onions, cucumber, chickpeas, parsley, mint, oil, lemon zest and lemon juice.
Season to taste with salt and pepper.
Cover and chill overnight to let flavors develop. Serve topped with hazelnuts.