Season the steak generously with salt and pepper on both sides. If the steak is cold, let it rest until it comes close to room temperature, 30 minutes to 1 hour.
In a 10- or 12-inch cast-iron pan or other thick-bottomed skillet, heat a slick of vegetable oil over high until shimmering and very hot. Add the steak and let sizzle energetically for 2 to 3 minutes, until a nice brown crust forms on the bottom side.
Flip and cook for an additional 1 to 3 minutes, until the steak reaches your desired doneness. An instant-read thermometer is tremendously useful for checking doneness — medium-rare will read between 130°F to 140°F, medium will read 140°F to 150°F, and medium-well 150°F to 155°F. (If you don’t have a thermometer, just use a spatula to press lightly on the center of the steak. At medium-rare, the steak will feel like your cheek, mostly soft but with some structure; at medium, the steak will feel like the fleshy part of your chin, still soft but with more resistance; at medium-well, it will feel like your forehead between your brows, fairly firm.)
Remove the steak from the heat and let rest for 5 to 10 minutes to let the juices distribute. Meanwhile, make the gireumjang sauce.
For the sauce, mix together the salt and pepper in a small bowl. Stir in the sesame oil until incorporated. If still cooking the steak, refrigerate until needed. (The sauce will keep for several weeks.)
6. Slice the steak and serve with gireumjang drizzled over top or on the side as a dipping sauce. Enjoy!