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.