In a small bowl, combine soy sauce, oyster sauce, hoisin sauce, sesame oil, rice vinegar, beef broth, sugar, garlic, and ginger to make the sauce. Whisk well and set aside.

Place the sliced flank steak in a bowl and toss with cornstarch until evenly coated. Let it sit for 5 minutes.

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

Add the remaining 1 tablespoon of vegetable oil to the skillet. Add the broccoli florets and stir-fry for 3-4 minutes until they are tender yet crisp.

Return the beef to the skillet with the broccoli. Pour in the prepared sauce and stir to combine. Cook for another 2-3 minutes, allowing the sauce to thicken and coat the beef and broccoli evenly.

Remove from heat and garnish with sesame seeds, if desired. Serve immediately over steamed rice or noodles.