Form the Onigiri: Wet your hands with water (or plastic wrap). Place one serving of rice in your hand and gently press it into a round (or triangle shape) using your palms and fingers.
Make the Miso Sauce: Mix the miso paste and mirin in a small bowl until smooth and well combined.
Grill the Onigiri in the Oven: Line a baking tray with parchment paper. Place the rice balls on the tray and brush the tops with miso sauce. Grill at 150°C (300°F) for 5-7 minutes until the surface turns golden.
Notes
You can also make these using a toaster oven or a frying pan. If using a frying pan, add a little oil to prevent sticking.
The recipe calls for spreading miso on one side, but you can coat both sides and grill them for extra flavor.
Be sure to press the rice firmly when shaping to keep the onigiri from falling apart during grilling.