How to Make It
Place rice in a colander, and rinse under cold water until water runs clear, about 5 times. Transfer rice to a medium saucepan, and add 1 1/4 cup water; bring to a boil over high. Reduce heat to low, cover and cook until rice is tender but not mushy, 12 to 15 minutes. Remove from heat; leave covered, and set aside at room temperature for 10 minutes. Transfer to a glass bowl when cooled.
Stir together rice vinegar, sugar, and salt in a microwavable bowl. Microwave on high until sugar and salt dissolve, about 1 minute. Add half of the vinegar mixture to the rice, and toss gently to coat. Set remaining vinegar mixture aside.
Stir together mayonnaise, Sriracha, soy sauce, lemon juice, and sesame oil in a medium bowl; add chopped tuna, and toss to coat. Add tuna mixture to rice, and gently toss to combine. Wet hands with remaining vinegar mixture, and form rice and tuna mixture into 1-inch balls, pressing to firmly pack. Transfer to a platter, and garnish with eel sauce, pickled ginger, wasabi paste, and furikake, if desired.