Special equipment: an onigiri mold
Combine the sushi rice with 1 1/4 cups water in a medium pot, cover, bring to boil, then reduce the heat and simmer for 10 minutes (or use a rice cooker and set to the white rice setting).
While the rice cooks, dissolve the salt in 1/2 cup warm water. Mix the miso, honey and 1 teaspoon sesame seeds in a small bowl.
Once the rice is cooked, fluff gently and allow to cool until you can handle it.
Moisten your hands or an onigiri mold with the salted water and get about 2 tablespoons rice compacted. Make a divet in the center of the rice ball and fill it with an umeboshi plum half and a teaspoon of miso paste. Cover with another 2 tablespoons of rice and dampen with more salted water. Compress to form a tight ball.
To pack in a lunch, roll the outside of the rice ball in sesame seeds or furikake and then wrap in plastic wrap. To eat right away, fold a strip of nori around the outside and eat while it's still crispy. Dip in soy sauce if using.
To pack a bento box: Fit one or two plastic wrapped onigiri in the box. Tightly pack the baby carrots, radishes, nuts or cheese cubes around them. In a second layer, pack fruit and dessert treats. The key to packing a Bento box is to pack it tightly so that portions don't slip and slide around. You can pack sheets of nori separately to wrap around the onigiri at serving time if desired.
Use your imagination when filling onigiri. As long as the filling is strongly seasoned, it will probably be good! Try drained canned tuna mixed with mayonnaise, smoked salmon or avocado cubes. Onigiri taste best at room temperature.