Place flank steak in the freezer for 20 minutes to firm up, then slice very thin against the grain into small strips. Set aside.
Bring a large pot of water to a boil. Blanch broccoli florets for exactly 2 minutes, drain, and run under cold water to stop cooking. Set aside.
Whisk soy sauce, brown sugar, cornstarch, sesame oil, hot water, and black pepper in a medium bowl until cornstarch dissolves completely.
Heat 1 tablespoon olive oil in a wok or large skillet over high heat. Sear half the beef in a single layer for 90 seconds without moving, flip and cook 60 seconds more. Remove to a plate and repeat with remaining beef and 1 tablespoon oil.
Reduce heat to medium. Add grated ginger and garlic to the pan and cook 30 seconds until fragrant.
Immediately pour in the sauce, bring to a simmer, and stir until thickened, about 2 minutes. Return beef to the pan and add blanched broccoli; toss to coat and cook 1–2 minutes until heated through.
Spread the mixture on a sheet pan and cool completely for about 20 minutes to prevent condensation in containers.
Portion into six glass meal prep containers, label with the date, and refrigerate. To serve, microwave 90 seconds (stir halfway); add a splash of water if sauce is thick. Top with sesame seeds if desired.