1. In a small bowl, combine soy sauce, sherry, sugar, vinegar, red-pepper flakes, and cornstarch. In a large skillet, heat 1 tablespoon oil over medium-high. In 2 batches, cook meat, undisturbed, until browned on one side, about 2 minutes per batch. Transfer to a plate.

2. Add 1 teaspoon oil and bell peppers to skillet and cook, stirring, until peppers are crisp-tender, 2 minutes. Add garlic and cook until fragrant, 30 seconds. Whisk soy sauce mixture and add to skillet, along with steak. Cook, stirring, until sauce thickens, 1 minute. Stir in sesame seeds. To serve, spoon beef mixture over rice and top with scallion greens, if desired.