2. Chop the avocado and sprinkle it with 2 tablespoons of lime juice to keep it from browning. Add the avocado and remaining lime juice to the bowl and toss to combine.
(This can be made in advance and stored in the refrigerator for up to 1 day.)
For the chicken:
1. In a small food processor, puree the yogurt, red onion, lime juice and cilantro to make a yogurt marinade.
2. Transfer the marinade to a shallow bowl or a plastic bag. Add the chicken and coat well with the marinade. Refrigerate for 1 hour or up to 8 hours.
3. Preheat the grill to medium-high.
4. Remove the chicken from the marinade, discard the remaining marinade and season the chicken with salt and pepper. Grill the chicken on both sides until it is cooked through, about 6 minutes per side.
5. Serve the chicken with tomato-avocado salsa.