Wash, pat dry and then thinly slice 1 pound of flank steak against the grain into 1/4-inch strips. Place the sliced beef in a bowl, marinate with 1/4 tsp black pepper and 1 tablespoon soya sauce. Toss with 1 tablespoon cornstarch and 1 tablespoon of brown sugar until evenly coated
In a small bowl, combine low-sodium soy sauce, oyster sauce, sesame oil, rice vinegar, remaining brown sugar and red pepper flakes
Place a large skillet on the stove and put 1 tablespoon of vegetable oil. Put the beef strips in the pan in a single layer. Fry for 2-3 minutes on each side
Now put the remaining 1 tablespoon of oil in the same pan. Put minced garlic and grated ginger. Let them turn golden brown, then put the sliced bell peppers and onion on the pan. Saute the vegetables for 3 minutes
Now put the fried beef in the pan again with vegetables. Pour the sauce mix and stir gently. Mix 1 tablespoon of cornstarch in ½ cup water and put the slurry in the pan. Cook everything for 5 minutes. Adjust salt and pepper if needed