1-1/2 pound flank steak
1-1/2 teaspoon kosher Salt
1/2 teaspoon freshly ground black pepper
1/4 cup plus 1 tablespoon soy sauce
1/4 cup canola oil; more for the grill
1/4 cup minced fresh ginger
1-1/2 tablespoons minced garlic
3 tablespoons rice vinegar
2 tablespoons Asian sesame oil
1-1/2 tablespoons light or dark brown sugar
2 teaspoons cornstarch
20 scallions (preferably thick ones), roots trimmed
1 tablespoon sesame seeds, toasted
Season the flank steak with 1 teaspoon of the salt and the pepper. Mix 1 tablespoon of the soy sauce, 1 tablespoon of the canola oil, 2 tablespoons of the ginger, and 1 tablespoon of the garlic in a large zip-top plastic bag. Add the steak and turn and massage it in the bag to cover it with the marinade. Refrigerate for at least 4 hours or as long as overnight.
Heat 1-1/2 tablespoons of the canola oil and the remaining 2 tablespoons ginger and 1/2 tablespoon garlic in a small saucepan over medium heat until the ginger and garlic sizzle steadily and just begin to brown around the edges, about 3 minutes Add 1/3 cup water, the remaining 1/4 cup soy sauce, and the rice vinegar, sesame oil, and brown sugar. Bring to a boil over medium-high heat. In a small bowl, whisk the cornstarch with 2 teaspoons of water and stir it into the soy mixture. Cook until it returns to a boil and thickens slightly, about 1 minute Remove from the heat and set aside.
Heat a gas grill to medium high or prepare a fire on a charcoal grill with a medium-hot and a low zone. Rinse the scallions but do not dry them. Toss the scallions with the remaining 1-1/2 tablespoons canola oil and 1/2 teaspoon salt.
Clean and oil the grill grates. Grill the steak (over the hotter zone if using charcoal), covered, until it has good grill marks, 5 to 6 minutes Flip and reduce the heat to medium if using a gas grill or transfer the steak to the cooler part of the charcoal fire. Cook, covered, until the steak is done to your liking, 4 to 5 minutes for medium rare (cut into the steak to check). Transfer to a large cutting board, brush with about a third of the sesame sauce, and let rest for 5 to 10 minutes
While the steak rests, clean and oil the grill grates, set the scallions on the grill (over the cooler zone if using charcoal), and cook until they have good grill marks, 2 to 4 minutes Flip and cook until they’re tender, 2 to 4 minutes Transfer to a large platter and drizzle with a couple of tablespoons of the sesame sauce.
Slice the steak thinly and serve with the scallions, a drizzle of the remaining sesame sauce, and a sprinkling of sesame seeds.