Start by preheating your grill or grill pan to medium-high heat. Season the chicken with salt and pepper and grill for 6-7 minutes per side until cooked through.
While the chicken is grilling, prepare the salad ingredients by dicing the avocado and mango, chopping the red onion, and rinsing the baby red butter lettuce.
In a small mixing bowl, whisk together the olive oil and white balsamic vinegar, adding salt and pepper to taste.
In a large salad bowl, combine the lettuce, diced avocado, diced mango, and diced red onion. Gently toss to mix.
Top the salad with the sliced grilled chicken, drizzle with dressing, and serve immediately.