Place thinly sliced beef in a mixing bowl. Add 1 teaspoon of minced garlic, ¼ teaspoon salt, ⅛ teaspoon black pepper, 1 teaspoon oil and ½ teaspoon cornstarch. Mix well and set aside.
Slice potato in half lengthwise, then slice into thin slices (about ⅛ inch thick). If you are not cooking right away, cover potato slices with water to prevent them from turning brown/gray. Cut tomato in half, then cut into thin slices or thin wedges.
Place a pan over medium heat and add enough oil to cover the bottom. Once the oil is hot, add potato slices, stir and cook briefly for about 10 seconds. Add 1 teaspoon of minced garlic, stir and cook until garlic is fragrant.
Add about 3 tablespoons of water to the pan to cook the potato slices. Move them around to let them cook evenly. Cook for about 5 minutes until potato is softened and there is no water left in the pan. If potato is not softened enough, you can add another tablespoon of water and continue to cook.
Add onion slices to the pan, then stir and cook for 30 seconds. Add tomato slices and continue to stir and cook until onion and tomato slices are soft, about 2 minutes. Check if potato is almost tender to your liking, then add 1-1½ tablespoons oyster sauce. Stir quickly to combine thoroughly, then set the stir-fried potato aside.
You can continue cooking with the same pan or use a clean pan. Place it over medium-high heat and heat some oil. Add the remaining minced garlic and sauté briefly until fragrant. Then add beef slices, then toss and stir constantly to sear the beef.
Once the beef is about 80% cooked, add the stir-fried potato to the pan and toss everything together. Then add 1 teaspoon fish sauce and continue to toss and cook until the beef is just cooked through and potato is tender to your liking. Adjust seasonings to taste.
Sprinkle black pepper, add scallion pieces then give everything a final stir. Transfer to a serving plate and serve right away with rice.