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 ribeye slices, partially freeze the steak for about 30-45 minutes before slicing. This makes it much easier to achieve uniform, paper-thin cuts, crucial for quick cooking and tender texture. While the recipe mentions 'starts to brown' for onions, aim for a deeper caramelization without burning for a richer, sweeter base. A hint of dashi (Japanese stock) instead of just water would add profound umami depth to the sauce. Always serve gyudon immediately over freshly steamed, hot Japanese short-grain rice for the best experience, as the residual heat from the rice will meld the flavors beautifully.