Soak the potatoes in salted water for 5 minutes. Drain and pat dry.
Prepare the sauce: Whisk together the soy sauce, rice vinegar, sugar, and red pepper flakes with 1 tablespoon of water in a small bowl.
Heat the vegetable oil in a 10" cast iron skillet over high heat for 5 minutes, or until the oil is shimmering. Add the potatoes and fry, turning every minute or so, until the potatoes are fork-tender (about 4 to 6 minutes).
Add the garlic, scallions, and Serrano pepper to the pan; saute for 60 seconds until the garlic is fragrant.
Pour in the prepared sauce and toss well. Cook, turning the potatoes frequently, for 1 to 2 minutes, or until the potatoes have soaked up the sauce.