Individual servings of Asparagus Mousse look particularly attractive when garnished. Reserve 4 cooked asparagus tips and set them aside at room temperature. Just before serving, place 1 asparagus tip on each serving.
Boil, steam, or bake the asparagus until tender. Drain and transfer to absorbent paper towels. Pat the stalks dry and cut into 1-inch lengths. Reserve 4 tips for garnish, if desired. Transfer to a blender or food processor and process until smooth. Add egg yolks, one at a time, then blend in tarragon, salt, nutmeg, and pepper. Pour into a large mixing bowl.
Preheat oven to 350°. Generously butter 4 ramekins or individual souffle molds.
In a separate bowl, whisk egg whites until soft peaks form and fold into the asparagus purée. Pour into prepared baking dishes and loosely cover each with aluminum foil. Arrange baking dishes in a shallow pan, then pour in boiling water to a depth of 1 inch.
Bake for 25 to 30 minutes or until the blade of a paring knife inserted into the center comes out clean.
Garnish with asparagus tips (if using) and serve immediately.