Steps

1. Remove the pit from 1 umeboshi and mash finely.
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2. Add 1/2 tsp sugar and 1/2 tbsp miso to the mashed umeboshi and mix well. (Key Tip: Adding sweetness and acidity as a flavor accent creates grilled rice balls that you won't get tired of eating.)
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3. Place 500g of rice in a heatproof container.
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4. Heat the rice in the microwave (600W for approx. 2 minutes) until piping hot. (Key Tip: Heating the rice makes it easier to break apart and season without mushing.)
   🕒 02:01
5. Add 12g salted kelp and 1/2 tbsp soy sauce to the warm rice and mix until evenly combined. (Key Tip: The umami from the salted kelp makes it delicious even without side dishes, so keep the seasoning light.)
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6. In a small bowl, mix 1 tsp soy sauce and 1 tsp mirin (sweet rice wine).
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7. Pour sesame oil (to taste) into a separate container.
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8. Prepare a frying pan.
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9. Lightly coat your hands with sesame oil.
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10. Roughly divide the seasoned rice into 4 portions.
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11. Lightly form one portion of rice into a triangle, creating an indentation in the center.
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12. Fill the indentation with the prepared plum miso mixture.
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13. Seal the plum miso with rice, then firmly shape the onigiri to prevent it from falling apart. (Key Tip: Grilled rice balls are meant to be grilled or frozen, so it's best to pack them firmly to avoid crumbling. Since miso can burn easily, make sure to seal it well with rice.)
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14. Lightly brush the surface of the shaped onigiri with sesame oil. (Key Tip: Applying sesame oil prevents sticking to the pan and gives it a glossy, appetizing appearance.)
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15. Repeat the process for the remaining rice balls and arrange them in the prepared frying pan.
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16. Heat the frying pan over medium-low heat and grill one side of the onigiri until golden brown.
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17. Once a fragrant brown crust forms, flip the onigiri.
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18. Grill the other side until it also develops a nice golden-brown color.
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19. Turn off the heat once both sides are browned.
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20. Using a brush, coat the surface of the onigiri with the mixed soy sauce and mirin. (Key Tip: Soy sauce can burn easily, so by turning off the heat and using residual heat, you can achieve a fragrant finish without burning. Mirin also helps to dilute the saltiness of the soy sauce.)
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21. Flip the onigiri, brush the other side with soy sauce and mirin, and grill over low heat for a fragrant char. It's done!
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22. For freezing, wrap the hot, freshly grilled onigiri tightly in plastic wrap. (Key Tip: Wrapping them while still hot prevents them from drying out and becoming hard.)
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23. Once cooled to a touchable temperature, store in the freezer and consume within one month.
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24. Frozen onigiri can be reheated in a 600W microwave for about 1 minute and 40 seconds for a delicious result.
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