To make salsa, in a bowl combine tomatoes, cucumber, avocado, scallions and garlic. Sprinkle with juice and zest from 1/2 of lime.
Preheat grill to medium. Brush chicken with oil and season with black pepper and cumin.
Cover the grill and let the chicken cook for 5-7 minutes, then use tongs to flip the breasts over. Grill for 5-7 minutes longer, or until a thermometer reads 165° Serve chicken, top with tomato, cucumber, avocado salsa