This is basically a common Japanese fast food where beef and caramelized onions are served on top of short-grain rice. Perfect when hot, cold, or room temperature. Good for school lunches on hot days.
For perfectly thin beef slices, partially freeze the ribeye for 20-30 minutes before slicing against the grain; this makes it much easier to achieve uniform, delicate pieces. When caramelizing the onions, don't rush the process in step 2; allow them to develop a deep golden color and sweetness before adding liquids, as this builds the foundational flavor of the dish. The sauce should be balanced – taste and adjust soy sauce or brown sugar as needed to suit your preference, keeping in mind the sweetness from the caramelized onions. Serve immediately over freshly steamed short-grain Japanese rice for the best experience.