Rinse the sushi rice under cold water until the water runs clear. Combine the rice, water, and salt in a medium saucepan. Bring to a boil over medium heat, then reduce to low, cover, and simmer for 18-20 minutes, or until the rice is tender and water is absorbed. Remove from heat and let it sit covered for 10 minutes.

In a small bowl, mix the rice vinegar and sugar until dissolved. Stir this mixture into the cooked rice and fluff with a fork. Allow the rice to cool to room temperature.

In a large bowl, combine the drained tuna, diced avocado, cherry tomatoes, cucumber, and green onions.

In a separate small bowl, whisk together the soy sauce, sesame oil, lime juice, and a pinch of salt and pepper. Pour this dressing over the tuna and vegetable mixture and gently toss to combine.

To serve, scoop a portion of the cooled rice onto a plate, top with the tuna and avocado mixture, and sprinkle with black sesame seeds if desired.