Season the rice

• Mix the cooked rice with the rice vinegar and salt.
• Let it cool slightly so it holds together.

Make the tuna filling

• Drain the tuna well and place it in a bowl.
• Stir in the mayonnaise and sriracha until combined.

Assemble the onigirazu

• Lay the nori sheet flat on a piece of plastic wrap.
• Spread half of the rice in a square in the center.
• Top with the tuna mixture and cucumber sticks, then cover with the remaining rice.

Wrap and shape

• Fold the nori corners over the filling to form a tight square parcel.
• Wrap firmly in the plastic wrap and press to compact.

Sear and slice

• Heat a frying pan over medium heat and sear the parcel on both sides until the nori is crisp.
• Unwrap and cut in half to serve.