Instructions

1. Cut 1 pound flank steak across the grain into 1/4-inch-thick slices. If the slices are long, cut crosswise into 2-inch long pieces. Toss the flank steak, 1 tablespoon cornstarch, 1 teaspoon packed brown sugar, 1 teaspoon soy sauce, and 1/4 teaspoon baking soda together in a medium bowl until the beef is evenly coated. Let sit while you make the sauce, no longer than 30 minutes (any longer, the baking soda will turn the beef mushy).
2. Stir 1/3 cup water, 4 minced garlic cloves, 2 tablespoons packed brown sugar, 2 tablespoons soy sauce, 1 teaspoon cornstarch, and 1/4 teaspoon red pepper flakes if desired together in a small bowl until combined.
3. Heat 2 tablespoons neutral oil in a large 12-inch skillet over medium-high heat until shimmering. Working in 2 batches, add the beef in a single layer and sear until deeply browned on both sides and just cooked through, about 1 1/2 minutes per side. Adjust the heat as needed to avoid scorching. Transfer all the beef to a bowl.
4. Reduce the heat to medium. Add 2 chopped medium scallions and stir-fry for 30 seconds. Stir the sauce again until the cornstarch is suspended, then pour into the skillet. Cook, stirring and scraping up any browned bits from the bottom of the pan, until slightly thickened, 1 to 1 1/2 minutes.
5. Return the beef and any accumulated juices in the bowl to the pan. Toss until the sauce coats the beef and looks glossy, about 30 seconds. Serve with rice.