Preheat your grill or grill pan over medium-high heat.
Brush the chicken breasts with olive oil and season them with salt and pepper on both sides.
Once the grill is hot, place the chicken on it. Grill for about 6-7 minutes on each side, or until the chicken is cooked through and has nice grill marks. The internal temperature should reach 165°F (75°C).
Remove the chicken from the grill and let it rest for a few minutes before slicing it into strips.
In a large mixing bowl, combine the mixed salad greens, cherry tomatoes, cucumber, bell pepper, and red onion.
Toss the salad with the balsamic vinaigrette dressing.
Divide the salad into four plates or bowls.
Top each salad with sliced grilled chicken, avocado, and crumbled feta cheese.
Serve immediately and enjoy.