Add sliced steak and cornstarch into a gallon-sized freezer bag. Shake the bag until all sides of the steak pieces are coated. Seal the bag, removing air.
In a quart-sized freezer bag, combine tamari or soy sauce, brown sugar, garlic, ginger, sesame seeds, sesame oil, and red pepper flakes.
Label a new gallon-sized freezer bag. Add to the labeled bag the sealed steak with cornstarch bag and sealed sauce bag, and add broccoli florets on top. Seal bags together and freeze.
Cook:
In a large wok or skillet, add 3–4 tbsp vegetable or canola oil over high heat. Pour steak into wok, stir-fry, separate pieces, and stir constantly until the majority of the pieces have some golden or crispy edges on them, for about 3–4 minutes. Add broccoli, cook 1–2 minutes until crisp-tender. Add sauce, and cook, stirring constantly for about 1 minute until sauce slightly thickens.