This is a traditional San Francisco recipe. The fresh basil is a regional favorite.

1/4 cup fresh basil leaves,
1 cup mayonnaise
1/2 cup sour cream
1/2 cup chopped parsley
3 scallions with tops, chopped
3 tablespoons vinegar
1/8 teaspoon dried tarragon
1 teaspoon Worcestershire sauce
1/2 teaspoon dry mustard
1/4 teaspoon freshly-ground black pepper
1 clove garlic
2 tablespoons chopped chives

Combine all ingredients in a blender and blend until smooth.