1. Mix 1/2 teaspoon salt, 1/4 teaspoon pepper, sugar, and 1/2 cup vinegar in nonreactive bowl. Put onion slices in colander in sink; pour boiling water over them. Add warm onions to vinegar mixture with enough cold water to cover. Let stand until onions are pink, about 15 minutes. Set aside.
2. Put parsley, tarragon, capers, and orange zest in small bowl with pinch of salt; stir in juice and 1 tablespoon vinegar. Whisk in mustard, then slowly whisk in oil. Adjust seasonings and set aside.
3. Bring 1 inch water to boil in soup kettle. Put asparagus in steamer basket, then carefully place basket in kettle. Cover and steam over medium-high heat until asparagus spears bend slightly when picked up, 4 to 5 minutes. Transfer asparagus to clean kitchen towel to dry.
4. Arrange asparagus on platter. Spoon vinaigrette over it. Lift 1/4 cup or so of onion rings from liquid and scatter over asparagus. Sprinkle diced egg white over asparagus, then push yolk through sieve so that it falls evenly over salad. Serve.