In a medium bowl, whisk together soy sauce, oyster sauce, and cornstarch. Add the thinly sliced beef steak to the marinade and let it sit for at least 15 minutes.

While the beef marinates, finely mince the garlic and ginger. Heat a large skillet over medium-high heat and add vegetable oil. Once the oil shimmers, add the minced garlic and ginger, sautéing for about 30 seconds until fragrant.

Add the marinated beef to the hot skillet, searing it without stirring for 1-2 minutes. Stir gently until browned but still slightly pink inside, about 2-3 more minutes.

In the same skillet, add the sliced bell peppers and onion. Stir-fry the vegetables for 2-3 minutes until tender-crisp.

Return the seared beef to the skillet with the vegetables. Sprinkle black pepper and drizzle in sesame oil, mixing thoroughly and cooking for an additional 2 minutes.

Remove from heat and top with chopped green onions and optional sesame seeds. Serve immediately over cooked rice.