Prepare the ingredients: slice the Onion thinly and separate the Thinly Sliced Beef so the strips will cook evenly.
Combine the simmering sauce: in a medium saucepan over medium heat, add Dashi Stock, Soy Sauce, Mirin, Sake, and Sugar. Bring to a gentle simmer and taste, adjusting sweetness or saltiness to your preference.
Add the sliced Onion to the simmering sauce and cook until the onion is translucent and softened, about 5–7 minutes.
Slide the Thinly Sliced Beef into the pot in a single layer as much as possible; simmer just until the beef is barely cooked through and tender, about 1–3 minutes depending on thickness.
Let the mixture simmer a little longer if needed so the sauce reduces slightly and coats the beef and onions, then remove from heat.
Divide the Cooked Japanese Short-Grain Rice among bowls and spoon the beef and onion mixture over the top, making sure to include some of the flavorful sauce.
Garnish each bowl with a little Beni Shoga, a scattering of thinly sliced Scallions, and a light sprinkle of Shichimi Togarashi for heat and aroma.
Finish each bowl by adding an Onsen Tamago on top if desired; break the egg and mix it into the hot beef and rice just before eating for a silky, rich finish.
Serve immediately while hot, and enjoy your comforting gyudon.
