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!
There is literally no easier recipe in the world, that tastes so good and takes only few minutes to cook, including rice cooking time! I always do it during hot summer days, because it's a no-brainer.
You can serve it as a main dish with more fancy toppings, or as a side dish, but also it's a great addition to bento or a small snack!