To prepare salsa: In a small bowl, combine the tomatoes, red onion, pepper and cilantro. 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. Prepare up to 1 day ahead of time. To prepare chicken : In a small food processor, puree the yogurt, red onion, lime juice and cilantro to make a yogurt marinade. 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. Preheat the grill to medium-high. 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. Serve the chicken with tomato-avocado salsa.