Separately, cook brown rice, according to pkg. directions.
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 heat.
In two batches, cook meat, undisturbed, until browned on only one side, about 2 minutes per batch.
Transfer to a plate.
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 toasted sesame seeds.
To serve: spoon beef mixture over rice and top with scallion greens if desired.