In a small bowl, stir together the soy sauce, rice vinegar, cornstarch, and sesame oil. Then stir in the garlic, ginger, black pepper, and red pepper flakes.
Heat a wok or skillet on high heat. Swirl in half the oil and add the beef in a single layer. Cook for about 1 minute and then stir. Cook 1-2 more minuets and remove from pan. It should be just cooked so it doesn’t get rubbery.
Add remaining oil to the pan. Add the onions and pepper and cook for 4 to 5 minutes.
Add the beef and sauce. Bring to a simmer and cook for 2-3 minutes until thickened. Season if needed and serve.
