1. Preheat the grill to medium-high heat.
2. Season the chicken breasts with salt and pepper.
3. Grill the chicken for 6-7 minutes on each side, or until fully cooked.
4. Let the chicken rest for 5 minutes, then slice it.
5. In a large mixing bowl, combine mixed greens, cherry tomatoes, cucumber, red onion, and feta cheese.
6. In a small bowl, whisk together olive oil, lemon juice, minced garlic, dried oregano, salt, and pepper.
7. Drizzle the dressing over the salad and toss to combine.
8. Top the salad with grilled chicken slices and serve immediately.