Trim the root of the shimeji mushrooms and separate them. Cut the king oyster mushroom in half and slice into bite-sized pieces.

Make small cuts in the connective tissue of the pork loin to prevent curling. Season both sides with salt and pepper, then coat with flour.

Heat salad oil in a frying pan over medium heat. Place the pork in the pan and cook for 3–4 minutes on one side until browned. Flip and cook for another 3 minutes. Remove from the pan.

Wipe the pan clean with a paper towel. Add butter and heat over medium heat. Sauté the mushrooms until tender, then add the sauce ingredients (sake, sugar, soy sauce, mustard) and simmer until thickened.