Slice the flank steak thinly against the grain into strips approximately 2 inches long. Place the strips in a bowl and coat evenly with the cornstarch by tossing gently.
In a mixing bowl, combine the orange juice, orange marmalade, soy sauce, rice wine vinegar, sesame oil, brown sugar, red pepper flakes, salt, and black pepper. Whisk until all ingredients are fully incorporated.
Heat the vegetable oil in a large skillet over medium-high heat. Add the cornstarch-coated steak to the skillet and cook, stirring occasionally, until the exterior is evenly browned, approximately 5 to 8 minutes.
Pour the prepared orange sauce into the skillet with the cooked beef. Stir the mixture continuously, allowing the sauce to thicken as it cooks. This process should take about 8 to 10 minutes.
Once the beef is fully cooked and the sauce has reached the desired consistency, remove the skillet from the heat.
Serve the Orange Beef over a bed of rice or noodles, garnished with sesame seeds. Enjoy immediately.