Yaki onigiri freeze extremely well, retaining their shape and flavor when packed in bento lunches and eaten at room temperature (important: reheat in the microwave before packing). Yaki onigiri are classic izakaya (pub) or bento food -- very nostalgic for us. I'm lucky my husband didn't eat all of them when I was making them!
First heat the grill to medium heat, place the onigiri on the grill, and don't move them at all for several minutes. Gently turn it over once the bottom is lightly browned as shown here. Grill the bottom half until browned, then turn down the heat to low.
With the heat now reduced to low, lightly brush the browned top with soy sauce and turn it over so that it heats through. Brush the other browned side with soy sauce, and turn it over again so that both sides have been grilled twice: once plain, and once with soy sauce. If you like, you can also brush the sides with soy sauce and grill those as well. The onigiri should now have a crisp outside crust.
It's now ready to eat, pack in your lunch, or freeze. To freeze, first wrap each individual onigiri in plastic wrap, freeze, then put them all in a freezer bag for longer-term storage (sucking the air out of the bag with a straw -- think of do-it-yourself FoodSaver vacuum-packing). To use a frozen yaki onigiri, it's important to reheat it first in the microwave before packing (on Cook until it's warm), otherwise the texture of the soft rice inside will be nasty. The crunchy exterior softens in the freezing/reheating, but otherwise tastes the same as when it's fresh.
My son had a similar bento today, but with pre-frozen onigiri rolled in sakura denbu (sweet, colored fish flakes) and red hana ebi (savory, colored fish powder).
Surprisingly, you can actually prepare onigiri in advance and stash them in the freezer. No, seriously, you can -- it's in Japanese-language bento books and I saw people do it when I lived in Japan. The trick is to use very fresh rice (that's moist and hasn't been sitting in the rice cooker for hours), wrap each onigiri individually before freezing, and after you take them out of the freezer be sure to heat them in the microwave until they're warm and soft again. If you thaw them on the counter or in the refrigerator the texture will be hard and nasty, so the microwave step is very important. (EDIT: If you're concerned about microwaving food wrapped with plastic wrap, unwrap the frozen rice, place it in a bowl, then cover the bowl with a lid, microwave-safe cover or plastic wrap that doesn't touch the surface of the food. Then microwave until warm.)
In this photo I put all of the freshly wrapped, warm onigiri (shaped in molds) on a metal pie plate to speed freezing.
After freezing, I put them in a labelled freezer bag and sucked all of the air out of the bag with a straw before sealing (like do-it-yourself FoodSaver vacuum packing). This helps ward off freezer burn.
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