In a medium bowl, combine the miso, 1/4 cup of the mirin, and the sake to make a soft paste. Spread about one-third of the paste (a heaping 1/3 cup) in a large pan or platter that will hold the steak. Lay two layers of cheesecloth over the miso marinade and set the steak on top. Cover with two more layers of cheesecloth. Spread the remaining miso marinade on top. Cover with plastic wrap, and refrigerate 5 to 10 hours, or overnight. (If marinating overnight, don’t leave the marinade on the meat longer than 10 hours; remove the steak from the marinade, wrap it in plastic, and refrigerate until you’re ready to cook.)
Remove the steak from the cloth, reserving 2 Tbs. of the marinade. If there is any miso paste on the beef, gently wipe it off with a paper towel.
Heat a large skillet on medium high and add the oil. When it’s hot, add the steak, browning it on one side for 3 to 4 minutes. Reduce the heat to medium low and cook for another 3 minutes. Turn the steak over, increase the heat to medium high, and brown on the other side for about 2 minutes. Reduce the heat to medium low and cook until medium rare, another 1 to 2 minutes. Check the doneness by pressing the meat with your fingers or by making a tiny cut into it (remember that it will continue to cook a bit off the heat). When it’s resilient outside but feels soft as you press deeper, it’s rare. A 1-inch-thick steak will take about 10 minutes to cook to medium rare.
Transfer the steak to a plate and cover loosely. Pour off any excess oil from the skillet, set the skillet over medium heat, and add 1/2 cup water and the remaining 1/4 cup mirin, scraping up any browned bits to help them dissolve. Add the reserved 2 Tbs. miso marinade, lower the heat, and cook, stirring, for 2 minutes. Remove the pan from the heat and add the brandy (if you’re comfortable doing this, light it with a match to burn off some of the alcohol; otherwise, skip this step). Using a sieve lined with cheesecloth, strain the sauce into a small saucepan. Simmer over medium heat until it’s slightly thickened. Cut the steak into 1/3- to 1/2-inch slices and serve drizzled with the sauce.