In a large bowl, combine the flank steak slices with soy sauce, garlic, ginger, and a pinch of salt and pepper. Marinate for at least 30 minutes, or up to 2 hours in the refrigerator for more flavor.
After marinating, sprinkle the cornstarch over the beef and toss to coat evenly.
Heat the vegetable oil and sesame oil in a large skillet or wok over medium-high heat.
Once the oil is hot, add the beef in a single layer. Cook for about 3-4 minutes without stirring until the beef is crispy and browned. Flip the beef and cook for another 2-3 minutes until fully cooked.
Add the red pepper flakes, honey, and rice vinegar to the skillet. Stir well to combine and cook for an additional minute.
Remove from heat and stir in the chopped green onions. Serve hot over rice or noodles.