Pat the chicken breasts dry, then rub with olive oil, garlic powder, oregano, salt, and black pepper. Grill over medium-high heat for 6–7 minutes per side until fully cooked (internal temp 165°F / 74°C). Let rest for 5 minutes, then slice.
Grate the cucumber and squeeze out excess moisture. In a bowl, mix Greek yogurt, grated cucumber, garlic, olive oil, lemon juice, dill, salt, and pepper until smooth and thick. Adjust consistency if needed.
Add lettuce or spinach, cherry tomatoes, cucumber, red onion, and olives to a large bowl. Gently mix in quinoa or farro if using.
Top the salad with sliced grilled chicken. Spoon tzatziki generously over the top or serve on the side. Finish with feta cheese and fresh herbs. Serve immediately or store components separately for meal prep.