Start by marinating the beef: In a bowl, combine the sliced flank steak, cornstarch, a pinch of salt, and pepper. Toss to coat evenly and let it sit for about 10-15 minutes while you prepare the other ingredients.
In a large skillet or wok, heat the sesame oil over medium-high heat. Once hot, add the marinated beef in a single layer and sear for 2-3 minutes undisturbed to form a crust.
Flip the beef and cook for another 2 minutes until browned but not fully cooked. Remove from the skillet and set aside.
In the same skillet, add the garlic and ginger. Sauté for about 30 seconds or until fragrant, then add the sliced bell peppers and green onions. Stir-fry for 3-4 minutes until the veggies are slightly softened.
Add the beef back to the skillet along with the soy sauce, hoisin sauce, rice vinegar, and red pepper flakes. Stir everything together and cook for another 2-3 minutes, until the beef is cooked through and the sauce is heated and thickened.
Adjust seasoning if needed, adding more salt or pepper to taste.
Serve immediately over cooked rice or noodles, and garnish with additional green onions if desired.
Notes
Adjust the red pepper flakes to control the heat level.