Marinate the chicken:
In a small bowl, whisk together balsamic vinegar, olive oil, mustard, garlic, salt, and pepper. Place the chicken fillets in a shallow dish and pour the marinade over them. Let sit for at least 10 minutes while you prepare the salad.

Cook the chicken:
Heat a grill pan or skillet over medium-high heat. Cook the chicken for 4–5 minutes per side, or until golden brown and cooked through. Let rest for a few minutes, then slice.

Prepare the salad:
In a large bowl, toss together salad greens, cherry tomatoes, cucumber, red onion, and avocado. Add cheese and basil if using.

Make the dressing:
In a small jar or bowl, whisk together balsamic vinegar, honey, mustard, olive oil, salt, and pepper until well combined.

Assemble the salad:
Place the salad on a large serving platter or individual plates. Top with sliced chicken and drizzle with balsamic dressing just before serving.

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