Slice the beef into thin strips against the grain.
Marinate the beef with soy sauce, oyster sauce, garlic, ginger, salt, and pepper for 10 minutes.
Heat vegetable oil in a large pan or wok over medium-high heat.
Add the marinated beef to the hot pan in a single layer.
Stir-fry the beef for about 3-4 minutes until browned and cooked through.
Remove the beef from the pan and set aside.
In the same pan, add a bit more oil if needed and toss in the mixed vegetables.
Stir-fry the vegetables for 2-3 minutes until they are tender-crisp.
Return the beef to the pan and mix everything together.
Drizzle sesame oil over the stir-fry, toss to combine, and serve hot, garnished with sesame seeds.