Heat sesame oil, garlic and ginger in a saucepan over medium heat. Add soy sauce, water brown sugar, vinegar, and red pepper flakes. Let boil for 10-12 minutes until it thickens slightly.
Toss the beef strips in cornstarch (I used a large ziplock bag, sealed it and shake until coated.) Shake off any excess cornstarch.
While sauce boils, Heat a large skillet over medium to high heat. Add 1 tbsp of olive oil. Add beef in small batches cook until brown, about 2 minutes. remove from pan and set aside on a paper towel. Repeat until all the beef is cooked. Beef doesn’t have to cook all the way through. it will finish cooking later in sauce.
Return all the beef to the pan and pour in the thickened sauce and toss to coat evenly.
Add green onions and stir. Serve with veggies and steamed rice or noodles.