This one is a real pickle — it's extremely simple, and doesn't look spectacular, but packs a serious punch of flavor and deliciousness. You can easily upgrade or downgrade the recipe based on your liking, use different ingredients, but as long as you make ground meat under a blanket of mashed potatoes, it's gonna be fine ;-)
Bento is not only rice, but also other ingredients, like... pasta. This recipe came to life when I had some uncooked beef left-overs and just wondered what to do with them. Cooking them as a part of bento was a great idea!
You can buy thin beaf slices in some grocery shops or butchers. They may be called minute steaks or quick steaks and are a great ingredient that you can use to make a quick donburi (a rice bowl with meat and vegetable toppings). It's so fast to prepare, that you can do everything in the time it takes to cook the rice!