We had dinner at Sumo Sam a few days ago and initially we just ordered 1 Gyudon and 1 California Maki. After realizing that the Gyudon was so delicious we ordered another one! It's really easy to make and the ingredients are available if you know where to look. I made this without Powdered Dash and Sake and it still turned out very well. I now have dashi and will do this again soon!
This is based on a video recipe i found in YouTube.
Sake, Mirin and Powdered dashi are available in a the grocery, just look for the Oriental/Asian section. Alternatively there are a couple of Japanese food stores around the metro.