Mix together the lemon juice, chervil, and egg yolk in a small bowl. Gradually whisk in the oil until the mixture is creamy and smooth. Cover and chill. Let stand for 3-5 minutes.
Trim the asparagus of any fibrous ends and steam it in a vegetable-steamer over simmering water until it is crisp-tender and turns a bright emerald green, no more than 5 minutes. Remove to a colander and place under cold running water until it is cool. Drain the asparagus and chill it until serving time.
Arrange asparagus spears on a salad plate and drizzle a small amount of dressing over them.