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 salmon with oil and season with black pepper and cumin. Grill 4-5 minutes per side or until internal temperature reaches 145 degrees F. Serve salmon, top with tomato, cucumber, avocado salsa