Smoked Mackerel Onigiri

I will probably never get sick of eating convenience store onigiri, straight out of the refrigerator case and stuffed full of canned tuna mixed with spicy mayo. But making these rice balls at home has a few strong advantages. For one, you get to eat them while the rice is warm, the grains sticking together gently but not yet solidified into place. Second, you can experiment with different fillings and mix-ins. Here they take the form of smoked mackerel (I encourage you to try these with oth