1Bring quinoa, 1/2 tsp salt, and 1 1/4 cups water to a oil in a medium saucepan over high heat. Reduce heat to medium-low; cover and simmer until the quinoa is tender, about 10 minutes. Remove from heat and let stand, covered for 5 minutes. Fluff with a fork.
2Meanwhile, whisk the lemon juice and garlic in a small bowl. Gradually whisk in olive oil. Season to taste with salt and pepper.
3Spread out the quinoa on a large rimmed baking sheet; let cool. Transfer to a large bowl; mix in 1/4 cup dressing. Add the cucumber, tomatoes, herbs, and scallions to the bowl; toss to coat. Season to taste with salt and pepper. Drizzle with the remaining dressing.