In a bowl, combine the sliced flank steak, soy sauce, and cornstarch. Mix well and let it marinate for about 10 minutes while you prepare the vegetables.

Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the marinated beef and stir-fry for about 3-4 minutes until browned. Remove the beef from the skillet and set aside.

In the same skillet, add the remaining tablespoon of vegetable oil. Add the broccoli, bell pepper, and snap peas. Stir-fry for about 3-4 minutes until the vegetables are tender-crisp.

Add the minced garlic and ginger to the skillet, stirring for about 30 seconds until fragrant.

Return the cooked beef to the skillet. Add the oyster sauce and sesame oil, stirring everything together until well combined and heated through, about 2 minutes.

Remove from heat and garnish with sliced green onions. Serve over cooked rice or noodles.