Nestle a large Ziploc bag in a bowl. Add all of the marinade ingredients, zip closed and massage the bag until well blended. Add the steak, flipping it to coat it in the marinade, and then seal it again tightly. Place the steak in the refrigerator and marinate overnight, or for a minimum of 10 hours. Don’t sweat the timing, I’ve marinated it for 5 hours and it’s still delicious. Take the steak out of the refrigerator 30 minutes before cooking to bring it to room temperature.

Heat the oven to 400 degrees. Set your rack in the center of your oven.

Take the steak out of the marinade and pat dry with paper towels. This is important: wet meat does not sear. Season on both sides with kosher salt. Heat your skillet on high heat until very hot. Add about 1 tablespoon of a high heat oil, like vegetable or olive oil. Get your iphone timer ready. Place the steak in the pan and sear undisturbed for 2 minutes. Don’t worry if your steak looks too big for the pan, it’ll shrink to fit within a minute. Flip and sear for an additional 2 minutes. Both sides should be well browned. Remove from heat.

Transfer the skillet inside the oven and cook for about 6 minutes for medium rare and 8 - 10 minutes for medium (depending on how thick your flank is).

Let sit 5 minutes on a cutting board. Slice the beef thinly against the grain. Serve garnished with scallion, sesame seeds and warm ginger miso sauce on the side.

Meanwhile, heat oil in a small saucepan over medium-high heat until shimmering hot. Add the ginger and garlic, and cook, stirring, until beginning to brown, about 30 seconds. Whisk in the mirin, miso, lime juice, vinegar, and Sriracha, and cook until slightly thickened, about 1 minute. Stir in the sesame oil, transfer to a heatproof bowl, and keep warm.