Another way to flavor onigiri is by not filling them at all but instead sprinkling the surface with plain seaweed flakes or, ...

Steaming hot, white rice can be hand-formed or made with an onigiri press, fully or partially wrapped with nori (or plain), and topped with ingredients like furikake, toasted sesame seeds ...

She describes onigiri as “Japanese rice balls made of steamed rice ... Finish off with sprinkles of furikake or a few bonito flakes. Fillings: Consider cooked and chilled medium-size shrimp, cooked ...