In a medium bowl, combine sliced flank steak or sirloin with soy sauce, oyster sauce, and cornstarch. Let marinate for about 10 minutes.

While beef is marinating, wash and slice the bell pepper, chop broccoli, and slice green onions.

In a large pan or wok, heat 2 tablespoons of vegetable oil over medium-high heat until shimmering.

Add marinated beef in a single layer and sauté for about 2-3 minutes on each side until browned and cooked through. Transfer to a plate and keep warm.

In the same pan, add garlic and ginger, sauté for 30 seconds, then add bell pepper and broccoli. Cook for about 5 minutes until vibrant and tender-crisp.

Return the beef to the pan, drizzle with sesame oil, season with salt and pepper to taste, and stir to combine for 2 more minutes.

Garnish with green onions and serve hot over cooked rice or noodles.