3 tablespoons red miso
1 tablespoon soy sauce
1 tablespoon chili-garlic sauce
1 tablespoon white sugar
2 teaspoons toasted sesame oil
2 teaspoons finely grated fresh ginger
2 medium garlic cloves, finely grated
1 1/2 pounds skirt steak, trimmed and cut crosswise into 5- to 6-inch pieces, patted dry
2 tablespoon unseasoned rice vinegar
5 teaspoons grapeseed or other neutral oil, divided
In a medium bowl, whisk together the miso, soy sauce, chili-garlic sauce, sugar, sesame oil, ginger and garlic. Measure 2 teaspoons of the mixture into a small bowl and set aside. Add the steak to the remaining mixture, turn to coat and marinate at room temperature for 15 minutes. Meanwhile, to the reserved 2 teaspoons, stir in the vinegar and 3 teaspoons of the grapeseed oil, then set aside.
Remove the steak from the bowl an pat dry with paper towels. In a 12-inch skillet over medium-high, heat the remaining 2 teaspoons grapeseed oil until barely smoking. Add half the steak in a single layer and cook without disturbing until well browned and the center of the thickest piece reaches 125°F for medium-rare, another 2 to 3 minutes. Transfer to a platter, then repeat with the remaining steak, using the fat in the pan. Tent with foil and let rest for 10 minutes.
Stir any accumulated juices on the platter into the reserved miso mixture. Cut the steak against the grain on the bias into thin slices and return to the platter. Serve with the miso sauce.