In a small bowl whisk soy sauce, water, brown sugar, garlic, ginger and red pepper until combined.
Slice the steak thinly against the grain into bite sized pieces and place in large bowl.Toss the sliced flank steak and 1/2 cup cornstarch until evenly coated, and set aside.
Heat oil in a large cast iron skillet or wok over medium high heat. Once its nice and hot, cooking in batches add beef and cook just until browned and slightly crispy, about 2-3 minutes. Remove the beef and set aside.
Add the sauce into the pan and beef, simmer until thickened about 3-5 minutes.
Serve with rice and top with green onions and sesame seeds.