Grate a quarter onion on the fine side of a box grater, juice and all, and stir it into the yakiniku sauce.
Put the beef in a plastic bag, pour in the marinade, and work it in by hand until every slice is coated. Push out the air, seal, and refrigerate for about 2 hours.
Heat a pan with a little oil until properly hot. Sear the beef in a single layer, in batches, about a minute per side, until browned at the edges and the sauce has caramelized onto the meat. Wipe out any burnt bits between batches.
If you use the leftover marinade, pour it into the hot pan at the end and let it bubble for at least 30 seconds so it cooks through.
Fill a bowl with hot rice, lay the beef on top, spoon over any pan sauce, and finish with toasted sesame seeds.