In a small bowl, mix together the red onion and lemon juice. Set aside. Moisten your hands with the non-stick hand solution. Take a tablespoon-sized oval of sushi rice and form it into a rectangle shape between the palms of your hands. If you squeeze it too much it will begin to loose its texture and shape, so be as simple and quick as possible when working with the rice.

Hold the filet of ahi in the palm of your left hand, and sprinkle the red onion mixture evenly over the ahi, then rest the rectangle of rice on top of the onions and filet. Use your right hand's thumb and index finger to gently squeeze the rice and tuna into the rectangle shape while pressing down on the top of the rice with the thumb of your left hand to secure the onions sandwiched in between. Flip the nigiri over so that the filet is on top and repeat the gentle squeezing process once more. Your nigiri is complete!