1 cup mayonnaise
1 tablespoon tarragon vinegar or white-wine vinegar
4 anchovy fillets, finely chopped
1 small garlic clove, minced
3 tablespoon chopped fresh flat-leaf parsley
3 tablespoon chopped fresh chives
1 tablespoon chopped fresh tarragon
1 cup sour cream
Coarse salt and freshly ground pepper
In a food processor blend 1/2 cup mayonnaise, the vinegar, anchovies, garlic, scallions, and herbs in a food processor until combined.
Whisk in remaining 1/2 cup mayonnaise and the sour cream; season with salt and pepper. Refrigerate 1 hour (up to 4 days).