This sauce hits so many flavor notes -- salty, sweet, sour, umami.
Using a ginger grater or the fine side of a box grater, grate ginger root. Fold a 12-inch square of cheesecloth into quarters. Place the grated ginger in corner of cheesecloth. Working over a medium bowl, tightly wring the ginger in order to extract the juice. Discard the cheesecloth and the remaining root.
Add the soy sauce, fish sauce, 3 tablespoons water, mirin, scallion, lemon juice, and vinegar to the ginger juice. Whisk to combine. Dipping sauce can be refrigerated in an airtight container up to 1 week.
Mirin is a sweet rice wine used in many Japanese dishes; look for it in Japanese supermarkets or in the gourmet-food section of your supermarket.