In a mixing bowl, combine the soy sauce, brown sugar, garlic powder, minced ginger, sesame oil, and red pepper flakes. Stir until the sugar is dissolved and well combined.
Add the thinly sliced steak to the marinade, tossing to coat. Sprinkle the cornstarch over the beef and mix until evenly coated.
Transfer the marinated beef into a freezer-safe bag or container. Add frozen broccoli on top. Seal tightly and label with the date and cooking instructions.
When ready to cook, thaw overnight in the refrigerator or cook directly from frozen. Heat vegetable oil in a skillet over medium-high heat. Add contents and stir-fry for 10-15 minutes until beef is cooked and broccoli is tender. Sprinkle with sesame seeds before serving.
