Cook the short-grain rice according to package directions, then let it cool slightly until warm enough to handle.
In a small bowl, mix the drained tuna, mayonnaise, sriracha, soy sauce, sesame oil, rice vinegar, and green onion until creamy and well combined.
Lightly sprinkle the warm rice with 1/2 teaspoon salt and gently fluff it.
Wet your hands with water and rub a small pinch of salt over your palms.
Place about 1/2 cup warm rice in your palm and gently flatten it.
Add 1 to 2 teaspoons of spicy tuna filling in the center.
Cover the filling with a little more rice, then gently shape into a triangle, ball, or oval.
Repeat with the remaining rice and filling.
Wrap each onigiri with a strip of nori.
Sprinkle with sesame seeds if desired and serve right away.