The filling uses relatively less dressing than you might see for a sandwich-style tuna salad or other fish salad, because it’s best to keep the fish on the drier side so that it doesn’t make the onigiri too soggy. Just be sure that the filling is a bit on the saltier side, so that it can balance out the rice once it’s all wrapped up.
If you’re looking for more onigiri ideas, here are a few fun recipes: (1) gochujang tuna melt onigiri, (2) spicy smoked salmon onigiri, and (3) adorable avocado Totoro soot sprite onigiri.
Find it online: https://tworedbowls.com/2016/05/03/spicy-salmon-onigiri/