To make celery tzatziki: Mix yogurt, celery, olive oil, lemon juice, garlic and walnuts, if desired. Season with salt and pepper, to taste. Stir well. Chill.
To assemble bowls: In two bowls, layer greens and cooked quinoa evenly. Top with grilled chicken.
Spoon 1/4 cup hummus on side of each bowl. Add olives, 2-3 tablespoons tzatziki and drizzle with olive oil.