1. In a small saucepan, bring water to a boil over high heat. Add quinoa and a pinch of salt; bring back to a boil. Reduce heat to low; cover and simmer about 6 minutes, until nearly all the water is absorbed and quinoa is tender and chewy but not soft.
2. Meanwhile, in a medium bowl, combine juice, oil, salt, and pepper. Fold in quinoa, scallions, cucumber, and herbs. Toss until thoroughly coated. Season to taste as desired.
3. Thinly slice tomatoes. Arrange on serving plates. Spoon about 1 cup quinoa over tomatoes. Crumble feta on top. Serve immediately.