Pour oil in frying pan. Drop in potatoes and fry until brown. Remove, drain and set aside. Drain all the oil except for 2 tablespoons. Combine cornstarch, water and fish sauce. Reheat the oil and add garlic. Stirn then add onion and stir briefly. Add beef and stir fry until beef is done. Add scallions, tomato and green pepper. Stir and add cornstarch mixture. Add potatoes, stir well to combine. Allow potatoes to reheat and then remove. Serve garnished with cilantro and black pepper.