Serve this tart, creamy dressing any time of the year with whatever vegetables are in season.
Puree mayonnaise, chives, anchovy, vinegar, lemon juice, salt, and pepper in a blender. Add parsley, tarragon, and water, and process, scraping down sides of blender occasionally, until mixture is smooth and pale green. Transfer to a dish, and serve with crudites.
Dressing can be covered and refrigerated for 2 hours or overnight. Stir before serving.