Start by preparing your dressing. In a blender, combine the mayonnaise, sour cream, fresh herbs (parsley, tarragon, chives), anchovies, vinegar, and garlic. Blend until smooth and creamy. Season with salt and pepper.
In a bowl, mix together the lettuce, cucumber, and avocado.
Add the blanched green beans to the salad mix for an extra crunch.
Spot the dressing over the salad, and mix to coat.
Garnish with extra fresh herbs if desired, and serve immediately.