Dipping Sauce: Mix the sour cream, mayonnaise, onion, lemon juice, capers, lemon zest, tarragon, onion powder, sugar and pepper together with 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 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.