Mix soy sauce and mirin in a small bowl to make the sauce.
In a medium bowl, mix steamed rice and 2 tbsp of the sauce mixture. (save 2 tbsp for later)
Divide the rice to 8. Wet hands with water and place 1/8 rice in one hand, and press and form into a triangle shape onigiri. Repeat and make 8.
Heat the grill at medium high heat and oil the surface of the grill. Place the onigiri rice balls on the grill. Cook one side for 2 to 3 minutes until lightly browned. Flip onigiri, and brush the sauce on the browned surface. After 2 to 3 minutes, flip and brush the sauce on the other side. Remove from the grill when both sides are browned.