Grate the onion and mix together with the soy sauce, honey, and mirin.
Season the beef with salt & pepper, then marinate it in the mixed ingredients, leaving it in the fridge, for about a day. This can be done in a ziploc bag.
Remove the meat from the marinade and wipe off any marinade left on it using a paper towel. Begin cooking the meat in a frying pan with some oil. Once all sides have cooked, wrap the meat in aluminum foil, then again in a towel. Allow it to sit for 1~3 hours.
Next, to make the sauce, add the leftover marinade from the ziploc bag into the frying pan to heat it. Add some water if the flavor is a bit strong.
Cut the roast beef into thin slices and cover in the sauce, then you are done.