Season the chicken breast with salt and pepper and cook it on a grill pan or skillet over medium-high heat for about 5–6 minutes per side until fully cooked. Once done, allow the chicken to rest for a few minutes before slicing it into thin strips.

Prepare the dressing by whisking together olive oil, red wine vinegar, dried oregano, salt, and black pepper in a small bowl. Mix until the dressing is smooth and well blended.

In a large salad bowl, combine the chopped romaine or mixed greens with the cooked quinoa, cucumber, cherry tomatoes, kalamata olives, red onion, and crumbled feta cheese. Toss lightly to distribute the ingredients evenly.

Top the salad with sliced grilled chicken and drizzle the dressing over everything. Toss gently to coat the ingredients and serve immediately while fresh.