1) Preheat the oven to 180C/Gas 4.
2) Heat a large nonstick oven proof skillet over medium-high heat. Add the olive oil and swirl to coat the skillet. Season the chicken breast with salt and pepper on the smooth side of each breast. Place in the skillet seasoned side down and cook without disturbing until brown and crisp, about 3 to 4 minutes. Season the top side of the chicken and flip. Cook 1 min over the burner; then transfer the skillet to the oven until the chicken registers 75 C on an instant-read thermometer, about 8 to 10 minutes more. Set chicken aside to rest for 5 minutes before serving.
3) Divide the romaine, celery, radishes, and feta among 4 plates. Drizzle about 3/4 of a tbsp of the olive oil over each salad, season with salt and pepper and serve with a lemon wedge.