Mix all of the marinade ingredients together in a bowl.
Pour half of the marinade into a sealable container and spread until the bottom is evenly coated. Place 2 beef steaks and 4 fresh shiitake mushrooms on top.
Spread the rest of the marinade over the top of the steaks and shiitake mushrooms, then place a piece of plastic wrap directly on the surface of the steak.
Seal the container and refrigerate for 24 hours.
Heat a pan on medium-high and add 1 tbsp cooking oil. While you wait, use kitchen paper to wipe the excess miso sauce from the surface of the steak and mushrooms. If marinade is left on the surface, it is likely to burn.
Once the pan is hot, add the steaks and mushrooms and reduce the heat to low.
When it's nicely browned on one side, flip it over and brown on the other side. Cook to your desired doneness.
Remove the steak from the pan and wrap with foil. Rest for 5 minutes.