Slice the beef thinly against the grain and toss it with soy sauce, cornstarch, and sesame oil.
Whisk the sauce ingredients together in a small bowl until smooth.
Heat a large skillet or wok over medium-high to high heat.
Add broccoli and stir-fry for about 2 minutes. Add water, cover, and steam for approximately 1 minute.
Remove broccoli and set aside.
Add oil to the hot skillet and sear the beef in a single layer for about 1–2 minutes per side.
Remove the beef once browned and cooked through.
Add garlic and ginger to the skillet and cook for about 20–30 seconds.
Return the beef and broccoli to the pan.
Stir the sauce again and pour it over the beef and broccoli.
Toss everything together for 1–2 minutes, until the sauce becomes glossy and thick.
Serve immediately over steamed rice or noodles.
Garnish with green onions and sesame seeds.