1.To make chervil vinaigrette, whisk ingredients in a small bowl until combined. Season to taste.
2.Trim asparagus; refrigerate.
3.Sift dry ingredients into a large bowl; stir in egg and soda water until just combined. The batter will still have lumps.
4.Heat oil in a large saucepan or wok. Dip asparagus in batter, in batches; drain excess batter. Deep-fry asparagus, in batches, until browned lightly and crisp. Drain on absorbent paper.
5.Serve asparagus with vinaigrette and mizuna.