Take a piece of plastic wrap, place 1 cup of cooked brown rice on the palm of your hand. You don't really need to flatten the rice. Just spread it out a bit. TIP: Any time that I work with rice, I always use plastic wrap. It prevents the rice from sticking everywhere and making a mess.
Take the ½ piece of karashi mentaiko and place it in the center of the rice.
Fold over the rice to cover the mentaiko, making a ball.
Now, the trick is to make a triangular rice ball by clasping your hands, right over left. The right hand forms a "mountain" shape while the left hand squeezes gently closed. Rotate the rice ball counter-clockwise and squeeze gently. Repeat this motion about 2 - 3 times.
Wrap with aji nori (seaweed).
Recipe by Bebe Love Okazu at https://bebeloveokazu.com/2010/04/27/karashi-mentaiko-onigiri-spicy-cod-roe-rice-ball/