Mix sauce and set aside.
Mix cooked sushi rice with 2 tbsp rice seasoning.
Line the triangle onigiri mold with sliced beef. Fill the mold with the seasoned rice. Press the rice into the mold.
Fold overhanging beef over the top of the mold.
Carefully remove the beef onigiri. Pan fry for two minutes each side. And brown all other sides.
When the onigiri is ready, add sauce and spoon it on top of the onigiri.