Rinse the quinoa under cold water. In a pot, combine quinoa with 2 cups of water. Bring to a boil, then reduce heat to low, cover, and simmer for 15 minutes until fluffy. Let it cool slightly.

In a small bowl, mix 1 tablespoon olive oil, lemon juice, minced garlic, salt, and pepper. Coat the chicken breasts with the marinade and let sit for 10 minutes.

Preheat the grill or grill pan to medium-high. Grill the chicken for 6-7 minutes per side until fully cooked and juices run clear. Remove and let rest for 5 minutes, then slice.

Chop cherry tomatoes, cucumber, red onion, and parsley. In a large bowl, combine the vegetables with cooled quinoa, crumbled feta (if using), and sliced grilled chicken.

In a small bowl, whisk together remaining olive oil, lemon juice, salt, and pepper. Drizzle over the salad and toss gently to coat.