In a bowl, combine sliced flank steak, 2 tablespoons soy sauce, and 1 tablespoon cornstarch. Mix until evenly coated. Let sit 15 minutes while you prep vegetables.

Place a large skillet or wok over medium-high heat. Add 1 tablespoon vegetable oil and heat until shimmering.

Add the beef in a single layer. Cook 2–3 minutes, stirring once, until browned but not fully cooked through. Remove beef to a plate.

Add the remaining tablespoon oil to the same pan. Add sliced onion and bell pepper. Stir-fry 3–4 minutes until they begin to soften and get a little color.

Stir in minced garlic and ginger. Cook 30–60 seconds until fragrant — don't let the garlic burn.

Return the beef to the pan. Pour in 1/4 cup beef broth and 1 tablespoon oyster sauce. Stir and cook 2–3 minutes until the sauce reduces slightly and coats the ingredients.

Drizzle 1 tablespoon sesame oil, taste, and season with salt and pepper as needed. Remove from heat and serve hot over cooked rice or noodles.