To Make Dipping Sauce: Mix the sour cream, mayonnaise, onion, lemon juice, capers, lemon zest, tarragon, onion powder, sugar and pepper together using a whisk. Mix well. Refrigerate until chilled, about 2 hours.
Cook the asparagus in a steamer basket over boiling water until tender when pierced with a fork, 6 to 8 minutes. Mix the ice, water, and kosher salt together in a large bowl. Remove the asparagus from the steamer and refresh it by plunging it into the ice water. Let sit until cold. Transfer to a paper towel-lined plate to dry. Refrigerate. Serve chilled with the dipping sauce.
Do-Ahead Tips: The dipping sauce can be prepared up to 2 days in advance amd refrigerated. The asparagus can be prepared 1 day in advance and refrigerated. Serve as directed.
Recipe courtesy of Cocktail Food, by Mary Corpening Barber and Sara Corpening Whiteford, published by Chronicle Books