Place the thinly sliced steak and cornstarch into a gallon-size freezer bag, seal, and shake or massage until the beef is evenly coated.
In a quart-size freezer bag combine soy sauce, brown sugar, garlic powder, minced fresh ginger, sesame seeds, sesame oil, and red pepper flakes; remove excess air and seal, then knead to mix the sauce.
Place the sealed sauce bag inside the gallon bag with the coated beef, then add the frozen broccoli on top; squeeze out excess air and seal the gallon bag for freezing.
Thaw the bag in a bowl of warm water until contents separate, or thaw overnight in the refrigerator.
Remove the sauce packet from the bag and set aside, then pour the broccoli into a bowl and set aside.
Skillet method: Heat vegetable oil over high heat in a large skillet until shimmering. Add the steak and stir-fry until mostly cooked through, about 3–5 minutes.
Add the broccoli to the skillet and cook until tender-crisp, about 2–3 minutes.
Pour in the reserved sauce and cook, stirring constantly, until the meat and broccoli are well coated and the sauce thickens slightly.
Instant Pot method: Layer the steak and broccoli in the bottom of the Instant Pot, add 1/2 cup beef broth or water, seal, and pressure cook on high for 3 minutes with a 10-minute natural release; remove lid and stir in the sauce, then let sit 5 minutes.
Crockpot method: Combine the thawed beef, broccoli, and sauce in the slow cooker, stir to combine, and cook on low for 4–6 hours.
After cooking by any method, stir well and serve immediately.
