Prepare the chicken breasts by seasoning them with salt and pepper.
Preheat the grill to medium-high heat.
Grill the chicken breasts for about 6-7 minutes on each side, or until the internal temperature reaches 165°F.
Remove the chicken from the grill and let it rest for a few minutes before slicing.
While the chicken rests, prepare the salad in a large bowl by combining the mixed greens, cucumber, bell pepper, cherry tomatoes, and red onion.
Add the sliced chicken on top of the salad and sprinkle with feta cheese.
Drizzle the balsamic vinaigrette over the salad and season with additional salt and pepper if desired.
Gently toss the salad to combine all ingredients evenly before serving.