To create this rice bowl, you will need to start by combining the soy sauce, mirin, sake, and sugar in a small bowl.
Set it aside so that the flavor combine while you are cooking the steak.
Heat a 10-inch iron skillet, and then add the oil to the skillet.
If you do not have an iron skillet to use, any skillet will work; just don't heat the pan before it is oiled.
When the oil is heated, place the steak in the skillet and cook it for seven or eight minutes.
Make sure to flip the steak so that it cooks evenly, and when the time is up, you should have a medium-rare steak.
If you want your steak a little more well done, you can add a minute or two to the total cooking time.
When the steak has a nice crust on both sides, add the garlic and the butter to the skillet.
Make sure to move the steak as the butter melts so that it coats the steak.
If you need, use a spoon to ensure that the butter sauce completely coats the steak.
When the steak is fully coated in the butter, remove it from the skillet so that it can rest.
It should be left alone for about eight minutes before you cut it.
While the steak is resting, pour the sauce that you made already into the skillet.
The burner should not be on, but the skillet should still be quite hot.
The sauce will begin to boil soon as it hits the pan.
Smash down, and stir the mixture until it stops boiling.
When the steak had a chance to rest, slice it diagonally into pieces that are relatively thin so that you can easily pick them up with chopsticks.
To serve, place about one cup of rice into the bottom of a bowl, pour a little bit of the sauce over the rice and garnish the dish with lettuce before adding steak to the top of the bowl.
Top with more sauce if desired and season with a bit of black pepper.