Peel a small knob of ginger and finely grate to yield 1 tablespoon.
In a medium bowl, whisk the white miso, rice vinegar, soy sauce, and granulated sugar until smooth, 30–60 seconds.
While whisking constantly, slowly stream in the vegetable oil until emulsified, 1–2 minutes, then whisk in the toasted sesame oil.
Whisk in the water until the dressing is smooth, pourable, and lightly clings to a spoon, 15–30 seconds.
Let stand 10 minutes to mellow the flavors, then use as a salad dressing, dip, or light marinade. Refrigerate in a sealed jar up to 5 days and stir before serving.