Heat up a frying pan on medium and add 1 tsp butter. Once the butter is melted, add 100 g salmon fillet to the pan and fry until it's cooked through.
Use a spatula break it up into smaller flakes, then add 1 tbsp Japanese soy sauce (koikuchi shoyu) to the pan. Mix well and cook until the liquid is gone.
Turn off the heat and transfer the cooked salmon flakes to a large heatproof bowl.
Add 350 g cooked Japanese short-grain rice, ½ tbsp toasted white sesame seeds and 2 tbsp finely chopped green onions to the bowl.
Mix well until the ingredients are evenly distributed through the rice.
Take approx 100-130g of the rice and shape it into a rounded triangle using your preferred method. (Make sure to sprinkle salt on your hands/plastic wrap/mold first. If using bare hands, submerge them in ice cold water and rub salt over them before shaping.)
Don’t handle the rice for too long, quicker is better. Once you’re happy with the shape, wrap it with nori.
Consume rice balls fresh as they don't store well at room temperature or refrigerated. For storage, freeze immediately after making.
When freezing, wrap rice balls in plastic while warm, then place in a freezer bag once cooled. Consume within two weeks for best quality (maximum storage time is one month).
To defrost, microwave frozen onigiri for 2 minutes at 500W with plastic wrap still on. Add 10-second intervals if needed until fully heated. Add nori just before eating.