* 2 tablespoons hoisin sauce
* 2 tablespoons rice vinegar
* 1 tablespoon cornstarch
* Coarse salt
* 1/2 teaspoon red-pepper flakes, plus more for serving (optional)
* 1 tablespoon plus 1 teaspoon vegetable oil
* 1 1/4 pounds flank steak, cut diagonally across the grain into 1/2-by-3-inch strips
* 4 cloves garlic, minced
* 2 scallions, sliced crosswise 1 1/2 inch thick, white and green parts kept separate
* White rice, for serving (optional)
1. In a small bowl, whisk together 3/4 cup water, hoisin sauce, vinegar, cornstarch, 3/4 teaspoon salt, and 1/2 teaspoon red-pepper flakes; set aside.
2. Heat 1 tablespoon oil in a 12-inch nonstick skillet over high heat. In two batches, cook steak until lightly browned, turning once, about 2 minutes per batch. Transfer to a plate.
3. Add remaining teaspoon oil to pan along with garlic and whites of scallions. Cook, tossing often, until fragrant, about 1 minute. Whisk hoisin mixture to combine; add to pan along with scallion greens.
4. Return meat to pan; cook, tossing to coat steak with sauce, 1 minute. Remove from heat. Serve immediately, over rice and sprinkled with red-pepper flakes, if desired.