Prepare the Salmon Filling: Flake leftover salmon into small pieces and mix it with Kewpie mayo and sriracha sauce in a bowl, adjusting the sriracha to your desired spice level.
Season the Rice: Microwave leftover cooked jasmine rice for 1 minute until warm, then mix in mirin and rice vinegar, adjusting to taste.
Shape the Onigiri: Wet your hands to prevent sticking, then take a handful of seasoned rice, flatten it, and create a small well in the center for the salmon filling. Encase the filling with rice and shape it into a triangle or oval, using an onigiri mold if desired.
Pan Fry and Wrap: Brush the shaped onigiri with sesame oil and pan-fry until golden brown and crispy. Wrap with nori strips for added flavor and easy handling.
Garnish (Optional): Sprinkle sesame seeds over the top of the onigiri for extra flavor and texture if desired.