To make the aspic: In a saucepan, mix the egg white, gelatin, the greens of leeks, celery, and parsley; pepper, salt, and stock. Bring to a boil over high heat, stirring until it comes to a boil. When it comes to a boil, stop stirring, and cook 5 seconds. Set aside, off the heat, and let rest undisturbed for 5 minutes. Strain through a sieve lined with wet paper towels.
