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

Sauté the beef: In a large skillet or wok, heat 1 tablespoon of vegetable oil over medium-high heat. Place the marinated beef in a single layer and stir-fry for 2 to 3 minutes until it’s browned. Remove the beef from the pan and set it aside.

Prepare the vegetables: In the same skillet, pour in the remaining tablespoon of vegetable oil. Add the onion, cabbage, and carrots, and stir-fry for 3-4 minutes until the vegetables start to soften. Add the minced garlic and grated ginger, and cook for an additional 1-2 minutes until fragrant.

Combine and season: Return the cooked beef to the skillet with the vegetables. Pour in the sesame oil, rice vinegar, and hoisin sauce (if using). Toss everything together and cook for another 2 minutes until well combined. Season with salt and pepper to taste.

Garnish and serve: Sprinkle the stir-fry with sesame seeds and garnish with chopped green onions. Serve hot over steamed rice or noodles for a complete meal.