Cut the beef into even pieces and place in a bowl.
Add soy sauce, dark soy sauce, cooking oil, starch, and black pepper. Mix well until evenly coated.
Marinate the beef for 30 minutes.
Dice the potatoes into small cubes.
Heat oil in a pan and fry the potatoes until fully cooked and lightly golden. Remove and set aside.
In the same pan, add a little oil and fry the marinated beef until fully cooked.
Add the fried potatoes back into the pan with the beef.
Pour in light soy sauce and oyster sauce.
Add black pepper and stir-fry everything evenly for 2–3 minutes.
Remove from heat and serve hot.