Pat the chicken breast dry, then season all over with salt, black pepper, garlic powder, and dried oregano. Keep seasoning consistent so the caramelized edges form evenly.

Heat a cast iron skillet over medium-high until it shimmers, then add the olive oil and swirl to coat. The pan should look glossy before the chicken touches down.

Place the chicken in the skillet and cook 6-8 minutes without moving it, until deeply golden with crisp edges. Flip once and cook another 6-8 minutes until the center is just cooked through and juices run clear.

Transfer chicken to a plate and rest 5 minutes, then slice into bite-size pieces with visible moist, tender interior. Resting helps keep the chicken juicy while you assemble the salad.

Add romaine lettuce, cherry tomatoes, red onion, and cucumber to a large serving bowl. Toss gently so the vegetables are evenly distributed.

Whisk mayonnaise, Dijon mustard, lemon juice, and apple cider vinegar until smooth and lightly thickened. Taste and adjust seasoning before combining.

Add sliced chicken to the salad and drizzle dressing over the top, then toss until everything is coated. Stop tossing once the leaves look glossy and the chicken is evenly covered.

Serve immediately so the lettuce stays crisp and the chicken edges remain caramelized. Add a final squeeze of lemon if you want a brighter bite.