Pre-heat grill on high, then lower to medium. Season chicken breasts with salt and pepper and place chicken on the grill for 5-6 minutes on each side. Chicken will be finished when juices run clear. Allow chicken to cool slightly.
Prep other ingredients: dice the avocado, cut the cherry tomatoes, chop the cilantro, and slice the onion and lime wedges.
Add mixed greens to a large bowl, then top with chopped cilantro and red onion. Evenly drizzle ranch dressing over the greens, then top with avocado and tomatoes.
Slice chicken breasts and add to the top of the salad. Squeeze a couple of slices of lime over the salad and add the remaining limes to the salad bowl.