Toast quinoa in saucepan over medium heat until it starts to make popping sounds. Add water and bring to a boil. Immediately reduce heat to low, cover and simmer until quinoa is tender, about 25 minutes. Remove from heat and let stand, uncovered, until cool. Fluff with a fork.
Meanwhile, whisk lemon juice and garlic in a small bowl. Gradually whisk in olive oil. Season dressing to taste with salt and pepper.
Transfer to a large bowl; mix in 1/4 cup dressing.
Add cucumber, tomatoes, herbs, and scallions to bowl with quinoa; toss to coat. Season to taste with salt and pepper. Drizzle remaining dressing over tabbouleh
Can be made 1 day ahead. Cover and chill. Bring to room temperature before serving.