Rinse and drain quinoa using mesh strainer. In a medium saucepan over medium heat, combine the quinoa, broth, and 2 tablespoons lemon juice. Bring to a boil then reduce to a simmer, cover and cook 10-15 minutes, until all broth is absorbed. Let cool.
Meanwhile, place diced tomatoes in a colander to drain excess moisture. Sprinkle with salt and let drain 10 minutes.
Combine cooled quinoa, drained tomatoes, cucumber, scallions, garlic, remaining lemon juice and olive oil. Season with salt & pepper to taste. Chill at least 1 hour. Stir in chicken, parsley & mint just before serving.