Marinate the sliced beef in soy sauce and cornstarch for about 10 minutes.
Heat 1 tablespoon of vegetable oil in a skillet over medium-high heat.
Add the marinated beef and cook until browned, about 3-4 minutes.
Remove the beef from the skillet and set aside.
In the same skillet, add the remaining oil and sauté garlic and ginger for 30 seconds.
Add the broccoli florets and stir-fry for about 2 minutes until bright green.
Return the beef to the skillet with broccoli.
Pour in oyster sauce and sesame oil, stirring to coat everything evenly.
Cook for an additional 2-3 minutes until the beef is fully cooked and the broccoli is tender-crisp.
Garnish with sesame seeds before serving.