To Prep
Cut and slice the onions in half and then thin lengthwise, from top to bottom (axis to axis).
Combine the soy sauce, sake, mirin and dashi into a bowl. Set aside.
To Cook
Heat the sesame oil in a large flat pan over high heat. Add the beef and cook, stirring frequently for about 2 minutes, until the beef browns slightly. Add the sugar and wait 10 seconds. Add the onion and cook, stirring constantly, for about 1 minute, until the onions begins to soften. Pour the liquid mixture into the pan. Simmer for about 5 minutes, until the ingredients cook through. While the gyudon is simmering, use a large spoon to skim off any scum and fat that appears on the surface.
While the ingredients are simmering, divide the rice among 4 large bowls. As soon as the gyudon is ready, spoon it over the rice and serve immediately. Season with shichimi togarashi and beni shoga, as desired.