Japanese onigiri are prepared with a wide variety of fillings, and fish is not at all a prerequisite. For example, in this recipe, it is not needed in any form. So if you were looking for something similar – catch a proven option.
Step by step cooking recipe
Boil the rice until cooked according to the instructions and mix with rice vinegar.
Finely chop the mushrooms with onions, fry until tender and season.
Shape the rice into patties, spread the filling and shape into onigiri triangles.