Slice pork into thin slices, about 1/8 inch thick. Add pork to a large mixing bowl, then add a third of the ginger and a half of the white scallion parts. Continue to add all the salt, sugar, cornstarch, water and some black pepper. Toss well, then stir in 1 tablespoon of oil and give everything another toss. Marinate for about 15 minutes.
While marinating the pork, gradually heat a sauté pan over medium heat. I recommend using a cast iron skillet. Once the pan is hot, add some cooking oil to coat the bottom. Let the oil warm up, then add the remaining julienned ginger and white scallion parts. Sauté until fragrant, around 30-40 seconds.
Add the marinated pork slices to the pan. Stir and cook until no longer pink on the outside, about 4 minutes. Important notes: We don't need the pan to be very hot as we are not searing the pork, but we don't want it to be too cold or the meat will be steamed. Keep the pan around medium heat and adjust as needed (you can raise it to medium-high heat for a bit after adding the pork to maintain the pan temperature). Also, if your pan is not large enough, cook in batch to avoid overcrowding and lowering the heat too much.
Add soy sauce and continue to cook until the meat is cooked through. Adjust seasonings to taste. Stir in the green parts of the scallions.
Sprinkle black pepper and transfer to a serving plate. Enjoy this Vietnamese scallion ginger pork stir-fry with steamed rice or plain congee.
Notes
Pork loin is not the same as tenderloin which is too tender and lean for this dish.