In a large saucepan, bring water to a boil. Add quinoa. Reduce heat; simmer, covered, until liquid is absorbed, 12-15 minutes. Remove from heat; fluff with a fork. Transfer to a large bowl; cool completely.
Stir spinach, tomatoes, cucumber, peppers and green onions into quinoa. In a small bowl, whisk dressing ingredients until blended. Drizzle over quinoa mixture; toss to coat. Refrigerate until serving.
