Season chicken evenly with salt and pepper.
Heat olive oil in a large skillet over medium-high heat. Once hot, add chicken and cook undisturbed for 4-6 minutes. Flip and repeat on the other side until the chicken reaches an internal temperature of 165°F.
Remove chicken from the skillet, let it rest for 5 minutes, and then slice.
In a large bowl, combine spinach, sliced avocado, cherry tomatoes, mozzarella balls, and basil leaves. Top with sliced chicken and dressing.
Toss well and serve