Place sliced steak in a plastic zipper bag. Set aside.

In a bowl, whisk together sesame oil, broth, cornstarch, soy sauce, sugar, garlic, and ginger. Pour into the bag with the steak and massage, then zip it shut and place in the fridge. Marinate for at least 30 minutes. 

Meanwhile, blanch the broccoli florets in a large pot of boiling water for 1 minute, then drain through a colander. Set aside.

When the beef has marinated to your liking, heat 1 tablespoon of oil in a large wok. Retaining the marinade, cook the beef in batches, searing on both sides for about 1 minute each side, until almost cooked through. Remove the beef to a plate and keep warm under foil. Wipe out the wok with paper towels.

Heat the remaining oil in the wok. Add the onion and blanched broccoli and stir fry for 3-4 minutes until crisp-tender. Return the beef and marinade to the wok and cook 1-2 minutes until the sauce is thick and beef is warmed through.

Serve over rice and garnish with sesame seeds, if desired.