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 tablespoons 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 until the sugar dissolves. Measure 2 teaspoons of the mixture into a small bowl and set aside. Add the steak to the remaining mixture, turn to coat and let marinate at room temperature for 15 minutes.
Meanwhile, to the reserved 2 teaspoons, add the vinegar and 3 teaspoons of the oil. Stir to combine and set aside. Remove the steak from the bowl and pat dry with paper towels.
In a heavy-bottomed 12-inch skillet over medium-high, heat the remaining 2 teaspoons grapeseed oil until beginning to smoke. Add half of the steak in a single layer and cook without disturbing until well browned, 2 to 3 minutes.
Flip the pieces and cook until the second sides are well browned and the center of the thickest piece reaches 125°F for medium-rare or 130°F for medium, 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.