Prepare a big bowl of cold water. Peel potatoes and place them in the bowl of water. Cut potatoes into matchsticks with 3/8-inch thickness. Also put all matchsticks in the bowl and soak in cold water for about 10 minutes.
While potato sticks are in cold water, slice beef thinly against the grain. Put beef slices in a bowl, add chopped garlic, oyster sauce, dark soy sauce, cornstarch, olive oil and black pepper, mix well. Set aside to marinate.
After 10 minutes, take potato sticks out of water, spread them on a large baking tray lined with a thick layer of paper towel. Place paper towel on top of potatoes, press to absorb excess water. Change paper towel as needed and let them dry, about 10 minutes.
While waiting for potatoes to dry, prepare all veggies: thinly slice onion, slice leeks into 1-inch long pieces.
Place a wok (or wok-shaped pan, heavy-bottomed pot) over medium high heat. Add at least 1 inch of oil. When oil reaches 350F, add potatoes. Cook in 2-3 batches depending on the size of your pan/pot and the amount of oil. Adjust heat to maintain the temperature of oil at 300F. Cook and stir frequently for about 5-6 minutes until fries are pale yellow and slightly bubbled. Transfer to a colander (or plate, tray) lined with paper towel to drain.
After all potato sticks have been fried the first time, start frying them the second time. Add more oil if needed, when the oil reaches 375F, add potatoes. Adjust heat to maintain the oil temperature at 350F, cook and stir frequently for 3-4 minutes until fries are golden brown and crispy to your liking. Transfer to a colander (or plate) lined with paper towel to drain, sprinkle a pinch of salt over fries. Place fries on a serving plate.
Remove oil from the wok/pan, wipe with paper towel and add new oil. With medium high heat, when oil is hot, add onion. Stir and cook for 2 minutes, then set aside.
Add oil if needed, wait for the wok/pan to be hot again then add beef. Spread beef slices into one thin layer, let them sit undisturbed for 20-30 seconds, then start stir-frying.
When beef is 90% cooked, add onion back to the pan, quickly mix and stir. Add fish sauce, black pepper, celery leaves and leeks, stir frying until beef is fully cooked. Turn off the heat, adjust seasoning to taste. Top the fries with stir-fried beef (or place beef next to fries), sprinkle cilantro over everything and serve hot.