Boil the potato cubes in boiling water for 5 minutes until half-cooked. Drain and set aside.
(Tip: Add a pinch of salt to the water for extra flavor.)
Heat oil in a wok or frying pan and sear the beef tenderloin cubes over medium-high heat until about 70% cooked. Remove and set aside. In the same pan, stir-fry the potato cubes until lightly golden and slightly crispy around the edges, then add the mixed bell pepper cubes and stir-fry until fragrant.
Return the beef to the pan and add 1/2 packet of Lee Kum Kee Black Pepper Chicken Sauce. Stir-fry over medium heat until the sauce thickens and evenly coats all the ingredients. Adjust seasoning if needed and serve.