Make the unagi sauce by combining tamari (or soy sauce), seasoned mirin (a type of Japanese sweetened rice wine), and sugar. Cook over medium heat to dissolve sugar for about 5 minutes until reduced to half the liquid amount (you should have about 1/3 cup).
If you're using vegan fish like I did in the video, slice into 1/4 inch pieces and panfry on each side until crisp. Cut into pea-sized cubes.
To make the onigiri, combine cooked rice, drained peas and corn, diced vegan fish, 1 tbsp mirin, 2-3 tbsp unagi (start with 2, taste, then add more if you want more), and 2 tbsp of furikake. Mix well until everything is evenly seasoned.
Use a piece of plastic wrap to form 1-inch rice balls, press just until the rice sticks to each other (avoid breaking the rice). Place in bento box on a sheet of nori, shiny side up. Alternatively you can use silicone mini-muffin molds to separate the onigiri.