Heat oil in small saucepan over medium-high heat until shimmering. Meanwhile, toss scallions, fresh ginger, garlic, white pepper, salt, and ground ginger together in heatproof bowl. Pour hot oil into bowl and stir to combine. Stir in soy sauce and vinegar. Let stand for at least 15 minutes to allow flavors to meld. Transfer sauce to airtight container and refrigerate until ready to use. (This sauce can be refrigerated for up to 3 days.)