Place couscous in a bowl and add 1 tablespoon olive oil and a pinch of salt.
Pour boiling water or broth over couscous, stir briefly, cover, and let sit for 5 minutes.
Fluff couscous with a fork to separate the grains.
Dice cucumber, halve tomatoes, finely chop red onion, and chop parsley.
In a small bowl, whisk lemon juice, olive oil, garlic powder, salt, and black pepper.
Add vegetables and parsley to the couscous.
Pour dressing over and gently toss until evenly combined.
Fold in feta cheese if using.
Let the salad rest for 5 minutes or chill before serving.