Position a rack in the center of the oven and heat the oven to 400°F.
Simmer the mirin, shallots, ginger, and garlic in an 8-inch skillet over medium heat until the mixture is syrupy and large bubbles start to form, about 5 minutes. Remove from the heat and whisk in 1 Tbs. of the grapeseed oil, the miso, and the hot sauce. Set aside.
Season the steaks generously with salt and pepper. Heat a 12-inch oven-safe skillet over high heat until very hot. Add the remaining 2 tsp. oil, swirling it until the pan is well coated. Cook the steaks, flipping once, until browned, about 4 minutes total. Using a pastry brush, spread the glaze evenly over the steaks, transfer to the oven, and roast until an instant-read thermometer inserted into the thickest part of the steak reads 130°F to 135°F for medium rare, about 4 minutes.
Transfer the steaks to a cutting board, let rest for 5 minutes, and then cut on the diagonal into 1/2-inch-thick slices. Pour any juice remaining in the pan over top and sprinkle with the scallions.