Bring a large pot of water to a vigorous boil. Add about a teaspoon of salt.
Snap off the ends of all the beans, leaving the tail on if desired
Cook beans and peas in separate batches, starting with the snow peas and ending with the haricots verts, just until color is brightened and they are crisp-tender. They should bend without snapping but not be at all limp, 30 to 60 seconds. Remove with a slotted spoon and immediately plunge into an ice-water bath.
Once beans have cooled completely, drain in a colander, shaking to remove as much excess water as possible. Spread them on a clean kitchen towel and gently pat away remaining moisture. Loosely wrap vegetables in the towel and chill for up to 2 hours.
To make dressing: Grate cucumber on the large holes of a box grater.
In medium bowl, mix together cucumber shallot, sour cream, buttermilk, and mayonnaise. Add fresh lemon juice, salt, parsley, chives, and cayenne pepper. Season with more salt and cayenne, if desired.
Arrange vegetables on a platter and serve with dressing for dipping.