In a small bowl, stir the soy and oyster sauces and sugar. Slice the beef ¼ inch thick against the grain, then cut the slices into ¼-inch strips. In a medium bowl, stir together the beef, 1 tablespoon of the soy sauce mixture and ¾ teaspoon black pepper.
In a large Dutch oven, boil 4 quarts water. Stir in the pasta and 1 tablespoon salt; cook, stirring occasionally, until al dente. Reserve ¼ cup of cooking water, then drain; wipe out the pot.
In the pot over medium-high, heat the oil until barely smoking. Add the beef in an even layer and cook, stirring once or twice, until lightly browned, about 8 minutes. Add the scallion whites, garlic and pepper flakes; cook, stirring, until fragrant, about 30 seconds, then stir in the tomato paste and pasta. Add the scallion greens; cook, stirring, until beginning to wilt, 1 to 2 minutes. Add the remaining sauce and the reserved pasta water; cook, stirring, until the pasta absorbs the liquid, about 2 minutes. Taste and season with salt and black pepper.
