In a bowl, combine the beef slices with soy sauce, oyster sauce, cornstarch, salt, and pepper. Let it marinate for about 15 minutes.
Heat a large skillet or wok over medium-high heat and add 1 tablespoon of vegetable oil.
Add the marinated beef to the skillet in a single layer. Cook for about 2-3 minutes without stirring until browned, then flip and cook for another 1-2 minutes. Remove the beef and set aside.
In the same skillet, add the remaining tablespoon of vegetable oil. Add the sliced onion, garlic, and ginger, and sauté for about 1 minute until fragrant.
Add the sliced zucchinis and bell pepper to the skillet. Stir-fry for 3-4 minutes until tender yet slightly crisp.
Return the cooked beef to the skillet, add sesame oil, and stir everything together for another 2 minutes to heat through.
Adjust seasoning with more soy sauce or pepper if desired.
Remove from heat and transfer the stir fry to serving plates. Garnish with sesame seeds and freshly chopped cilantro or green onion.