Prepare all ingredients by slicing the beef and vegetables.
In a bowl, combine soy sauce, oyster sauce, and cornstarch.
Heat vegetable oil in a large skillet over medium-high heat.
Add the sliced beef and stir-fry for 3-4 minutes until browned.
Remove beef from the skillet and set aside.
In the same skillet, add garlic and ginger; sauté for 30 seconds.
Add mixed vegetables and stir-fry for 3-5 minutes until tender-crisp.
Return the beef to the skillet and pour in the sauce mixture.
Toss everything together and cook for an additional 2 minutes.
Drizzle with sesame oil and garnish with green onions before serving.