Drizzle the chicken with oil and season with salt and pepper. Grill the chicken on both sides to an internal temperature of 160F / 71C. Set aside to rest.
Optional: toast the bun on the grill.
Layer the bun with lettuce, tomato, grilled chicken, and avocado slices. Top with the remaining bun.
Portion the mixed greens. Drizzle with olive oil, balsamic vinegar, salt, and pepper.
Serve the sandwich with a side of mixed greens and sweet iced tea.