**Marinate the chicken**
In a mixing bowl, whisk together olive oil, lemon juice, oregano, garlic, salt, and pepper. Add the chicken, turning to coat. Cover and marinate in the refrigerator for at least 30 minutes or up to 2 hours.
**Cook the quinoa**
In a medium saucepan, combine quinoa and water (or chicken broth). Bring to a boil, then reduce heat to low, cover, and simmer for 15 minutes or until the quinoa is tender and the liquid is absorbed. Fluff with a fork and set aside.
**Grill or sauté the chicken**
Heat a large skillet or grill pan over medium-high heat. Add the marinated chicken and cook for 5–7 minutes per side, or until fully cooked and the internal temperature reaches 165°F (74°C). Let the chicken rest for a few minutes before slicing.
**Make the tzatziki sauce**
In a small bowl, combine Greek yogurt, grated cucumber, olive oil, lemon juice, garlic, and salt. Mix until smooth and creamy.
**Assemble the bowls**
Divide the cooked quinoa among serving bowls. Top with sliced chicken, cherry tomatoes, cucumber, red onion, olives, and feta cheese. Drizzle with tzatziki sauce and garnish with fresh parsley or dill, if desired.
**Serve**
Serve immediately and enjoy this fresh, wholesome meal.