Read Makiko's entire article as she also give you a detailed how-to on making umeboshi onigiri. The pictures below are of our family favorite, bonito filled onigiri, but you can fill these with just about anything from last nights leftover fish, marinated mushrooms, or pickles. As long as the filling is on the salty side, most anything can be used. These are not dipped in soy sauce, so the only salt is in the center or on the outside of the rice. Have fun experimenting until you find the filling that you like.
When we went to Japan last April, I had the opportunity to hone my technique with my mother-in-law when we made onigiri for our day trip to Kamakura (pictured above) as well as to and from the airport. She is an expert at it and had three made by the time I had finished one. I think my youngest has been missing her onigiri, as he has been asking for it a lot lately. Either that or he has been seeing them on the news footage lately which has sparked his craving.
Bonito Filling for Onigiri:
1 cup (large handful) of bonito shavings
1/2 tsp/5ml wheat free tamari
1/2 tsp/5ml mirin
Mix ingredients in a bowl, using half for each rice ball.
I got this tip from Makiko too: place plastick wrap in the bowl first. You can use it to help shape the ball in a later step.