Mix ketchup and oyster sauce in a 1:1 ratio for about ¾C +/- total volume.
Add oil to wok, add potatoes and tool until half done.
Add onions, black mushrooms. Cook until onions translucent. Remove.
Add more oil to wok. Add half of the garlic, stir fry. Add half of the mushrooms, salt and pepper to taste. Cook to release liquid and evaporate.
Add half of oyster/ketchup mixture, mix to incorporate liquid. Add to potatoes. Repeat with remaining ingredients except scallions.
Finally, add scallions. Mix well. Chill.