In a large skillet or pot, combine all the SAUCE ingredients and bring it to a boil.
Add the sliced beef and allow the sauce to return to a boil. Cover and reduce the heat to medium-low. Simmer until the beef is almost tender, about 10 minutes.
Skim off the impurities with a fine mesh strainer.
Add the onions and continue to simmer covered until the onions are translucent, about 5 minutes.
Serve over a bowl of rice and sprinkle shichimi togarashi, sesame seeds, or scallions, or all 3 like I did! :) Serve Gyudon immediately.