So rather than just daydream about it, I checked out some recipes online and came up with this version. Most recipes call for boiling or stewing the beef & onion slices in a soy-mirin mixture, but I opted to stir fry the onions & beef instead to achieve a richer flavor. Really quite simple to make and, if I say so myself, even better than Yoshinoya's...
1 tsp. sesame oil
1/4 cup sake
Shichimi Togarashi (Japanese 7-spice blend) - optional
Red pickled ginger (Kizami Shoga).