1 cup good mayonnaise
1 cup chopped scallions, white and green parts (6 to 7 scallions)
1 cup chopped fresh basil leaves
1/4 cup freshly squeezed lemon juice (2 lemons)
2 teaspoons chopped garlic (2 cloves)
2 teaspoons anchovy paste
2 teaspoons kosher salt
1 teaspoon freshly ground black pepper
1 cup sour cream

Place the mayonnaise, scallions, basil, lemon juice, garlic, anchovy paste, salt and pepper in a blender and blend until smooth. Add the sour cream and process just until blended. (If not using immediately, refrigerate the dressing until ready to serve.)