Combine the stock or dashi and teriyaki sauce with the sugar (if using) in a medium saucepan and bring to the boil.
Add in the onion and simmer for 2-3 minutes until the onion has softened.
Add in the beef and stir until the beef is just cooked. Season with salt if necessary.
Fill 4 bowls with rice, flattening the top of the rice, and then add the beef mixture on top, allowing a little of the stock to soak into the rice.
Sprinkle over a little Japanese seven spice and spring onion if you like, and serve with pickled ginger.