Cook 2 cups of rice in a rice cooker or on the stove according to package directions.
Once complete, turn off rice cooker and allow to cool.
Measure out mirin and soy sauce with dash of sesame oil if desired.
Add 3 Tbsp of seasoning to rice, while reserving 1 Tbsp for brushing.
Mix rice until seasoning is evenly distributed.
Using a large pan, measure out a piece of parchment and cut corners.*
Heat pan on medium heat.
Prepare a cutting board and wet with water to avoid sticking (alternatively, wrap with plastic wrap)
Using wet hands or a large piece of plastic wrap, take a handful of rice (1/2 cup worth) and press firmly into a disk or triangle shape. Press harder than you think necessary if you don’t press hard enough the riceballs may fall apart.
Once you have a few onigiri ready to go add to your pan and cook for at least 3-4 minutes on each side. Monitor closely to avoid burning, once the first side is finished flip, and cook the second side. After flipping, you can brush with additional sauce.
Repeat for each ball of rice and serve while hot.
You can also wrap with a small piece of nori (dried seaweed) for more flavor and texture if desired.
*If using a non-stick pan you may not need parchment.
You can also use a toaster but you may need to watch closely and also shorten the cooking time to avoid burning.