Preheat the oven to 350 degrees F. Spread the walnuts on a baking sheet and bake until lightly toasted, 5 to 7 minutes. Let cool, then coarsely chop.
Make the dressing: Whisk the vinegar, mustard, sugar, 1 1/2 teaspoons salt and 1/2 teaspoon pepper in a large bowl. Gradually whisk in the olive oil until well blended, then whisk in the sour cream.
Add the cabbage, cilantro, dill, tarragon, scallions, grapes and toasted walnuts to the bowl; toss to coat with the dressing. Cover and refrigerate 1 to 4 hours before serving.
Photograph by Yunhee Kim
Courtesy of Food Network Magazine