Live in a Japan for any longer than about a day and you will run smack into Yoshinoya, or some other variety of gyudon, probably best described as simmered beef served on top of steamed rice. Ifyou discount Curry Rice, it is probably the most popular fast food in Japan. I pretty much guarantee that if you make it at home it will be better, and it’s really not that hard.
Put dashi, soy sauce, sugar, mirin, and sake in a large pan and bring to a boil on medium heat. Add onion slices and simmer for a few minutes or until softened. Add beef in the pan and simmer for a few minutes. Serve hot steamed rice into individual deep rice bowls. Put simmered beef on top of the rice. Top with some benishoga if you would like.
Good luck, and happy dining!