In a large bowl or resealable bag, combine the chicken breasts with olive oil, balsamic vinegar, minced garlic, dried oregano, thyme, coarse salt, and ground pepper. Marinate for 30 minutes to 1 hour in the refrigerator.

Preheat your grill or skillet to medium-high heat, around 375°F (190°C). Cook the marinated chicken for 6-7 minutes per side until it reaches an internal temperature of 165°F (74°C). Let it rest for 5 minutes before slicing.

In a large mixing bowl, combine the spring green mix, mozzarella balls, halved cherry tomatoes, fresh basil leaves, and diced avocado (if using). Toss gently.

In a small bowl, whisk together additional balsamic vinegar, olive oil, Dijon mustard, salt, and pepper until harmonious and slightly thickened.

Drizzle the dressing over the salad and toss gently until well-coated. Top with sliced grilled chicken and serve immediately.