Preheat the grill to medium-high heat.
Season the chicken breasts with salt, pepper, and a drizzle of olive oil.
Grill the chicken for about 6-7 minutes on each side until fully cooked.
Remove chicken from the grill and let it rest for 5 minutes.
In a large bowl, combine mixed greens, tomatoes, cucumber, bell pepper, and red onion.
In a small bowl, whisk together olive oil and balsamic vinegar.
Slice the grilled chicken into strips.
Top the salad with sliced chicken, diced avocado, and crumbled feta cheese.
Drizzle the dressing over the salad and toss gently to combine.
Serve immediately and enjoy your fresh grilled chicken salad!