Place the rice in a colander, and rinse it under cold water. Do this five times, or until the water runs clear. Transfer the rice to a medium saucepan, and add 1 1/4 cups of water, bringing it to a boil over high. Reduce heat to low, cover the pan, and cook the rice until it's nice and tender (but not mushy). Should take about 12 to 15 minutes. Remove the pan from the heat and leave it covered. Set it aside at to cool at room temperature for 10 minutes. When it's cool, transfer it to a glass bowl.

Stir together the rice vinegar, sugar, and salt in a microwave-safe bowl. Microwave on high until the sugar and salt dissolve, about 1 minute. Add half of your vinegar mixture to the cooled rice, and give it a gentle toss to coat. Set the remaining vinegar mixture aside.

Stir together mayonnaise, Sriracha, soy sauce, lemon juice, and sesame oil in a medium bowl. When it's mixed, add the chopped tuna, and give it a toss to coat. Add tuna mixture to rice, and toss it again to combine. Wet your hands with the remaining vinegar mixture, and make 1-inch balls with the rice/tuna mixture, giving them a firm press to pack. Transfer the balls to a platter, and garnish with eel sauce, pickled ginger, wasabi paste, and furikake, if desired (which it totally should be).