In large bowl, whisk together soy sauce, sesame oil, onion, scallions, and pinch salt. Toss with tuna and refrigerate until ready to use. In small bowl, toss cucumbers with vinegar, sugar, sesame seeds ,and pinch salt. Let stand 5 minutes. Serve tuna, marinated cucumbers, and avocado over rice if desired. Sprinkle with additional black sesame seeds if desired.