In a food processor, combine the rice vinegar, miso, mayonnaise, lemon juice, ginger and sugar and process until completely smooth. With the machine on, add the grapeseed oil in a slow and steady stream. Season the dressing with salt and pepper. Pour 1/2 cup of the dressing into a large bowl and refrigerate the rest for another use.

Add the green and red cabbage, carrots, radishes and scallions to the bowl and toss to coat thoroughly with the dressing. Serve slightly chilled or at room temperature.