Prepare for Onigiri: Remove the pit from an Umeboshi (pickled plum). In a small bowl, mix Katsuobushi (bonito flakes) with soy sauce.
Wet your hands with water and sprinkle them with salt. Place some rice in one hand and press it to form a triangle shape. Make a small well in the center of the rice ball and add a filling. Close the well, reshape the rice, and wrap it with a sheet of roasted seaweed.
Prepare side dishes: Make Rolled Chicken with Vegetables following the recipe.
Boil an egg, cut it in half, and sprinkle with salt and black sesame seeds.
Blanch the broccoli, then mix it with ground sesame seeds, soy sauce, and mayo.
Assemble Bento: Arrange the Onigiri rice balls and side dishes, along with sliced cucumber and cherry tomatoes for a splash of color.