In a bowl, combine the soy sauce, sugar, cornstarch, black pepper and the 1 teaspoon of cooking oil. Add the beef slices and mix to coat well. Let marinate 10 minutes at room temperature or up to overnight in the refrigerator.
Heat a wok or large fry pan over high heat until a bead of water sizzles and evaporates upon contact. Add the remaining 1 tablespoon of cooking oil and swirl to coat.
Add the beef slices to the wok in a single layer, laying out the slices all around the surface of the wok or pan. Fry 30 seconds, flip and fry another 30 seconds to 1 minute, depending on thickness of your steak, until just cooked through. Dish out, keeping as much sauce and oil in the wok as possible.
Turn the heat to medium and add the red onions and the garlic. Fry for 1 minute. Add the nectarine slices and let those get nice and warm, about 1 minute. Add the beef back into the wok, add the oyster sauce and toss to coat.