Prep Work: Roast walnuts in skillet. Cut or shred cabbage if needed. Chop cilantro, tarragon, onions and grapes; store with cabbage.
Roast walnuts by cooking them in a dry skillet on medium to high heat stirring often until darkened in color, approx. 3-5 min. Push pan off of heat and let nuts cool.
Meanwhile, whisk vinegar, mustard, sugar and 1 tsp. each salt and pepper (for yield of 6) in salad bowl. Whisk in olive oil and sour cream.
To this dressing add cabbage, cilantro, tarragon, onion, grapes and toasted walnuts. Toss and refrigerate for 1-4 hours for flavors to blend.
(adapted from Food Network Magazine July/August 2011 "Creamy Coleslaw with Grapes and Walnuts")