Place 1 cup mayonnaise, 1 cup scallions, 1 cup basil, ¼ cup lemon juice, 2 teaspoons chopped garlic, 2 teaspoons anchovy paste, 2 teaspoons salt and 1 teaspoon pepper in a blender and blend until smooth. Add 1 cup sour cream and process just until blended. (If not using immediately, refrigerate the dressing until ready to serve.)