1/2 lb green beans, trimmed
1/3 cup coarsely chopped fresh flat-leaf parsley
1/4 cup mayonnaise
1/4 cup sour cream
1 1/2 teaspoons red-wine vinegar
1/2 teaspoon fresh lemon juice
1/2 teaspoon anchovy paste
1/4 teaspoon salt
1/4 teaspoon black pepper
Cook beans in a 6- to 8-quart pot of boiling salted water, uncovered, until just tender, 6 to 8 minutes. Drain in a colander and immediately transfer to a bowl of ice and cold water to stop cooking. When beans are cool, drain in a colander and pat dry.
Purée parsley, mayonnaise, sour cream, vinegar, lemon juice, anchovy paste, salt, and pepper in a blender until smooth. Transfer to a bowl and toss with beans.