Bring quinoa, 1/2 teaspoon salt, and 1 1/4 cups water to a boil in a medium saucepan. Reduce heat, cover, and simmer until quinoa is tender, about 10 minutes. Remove from heat and let stand, covered, for 5 minutes. Fluff with fork and cool.
Whisk lemon juice and garlic in a small bowl. Gradually whisk in olive oil. Season wo taste with salt and pepper. Mix 1/4 cup dressing with quinoa.
Add cucmber, tomatoes, pepper, herbs, and scallions and toss to coat with remaining dressing.. Season to taste with salt and pepper. Serves 6 as side and 10 as main course.