Add onion, garlic, celery and carrot to mixer bowl, chop for 3 sec/speed 9/MC on.
Add butter and sauté for 5 min/100°C or 212°F/speed 1/MC off.
Add water, salt, pepper and bay leaf to mixer bowl, put chicken into steaming tray and steam for 60 min/steaming temp/speed 2.
Top up with water and steam for a further 20 min/steaming temp/speed 2.
Remove chicken from steaming tray and when cooled enough to handle, roughly remove the meat. Keep warm, covered, in a thermal pot.
With a fork, whisk together lemon juice and egg yolk in a measuring jug. Take 1 cup of stock from the mixer bowl and slowly whisk it into the lemon juice and yolk mixture with a fork.
Blend stock for 20 sec/speed 9/MC on increasing speed gradually.
Set timer for 1 min/speed 3, and slowly drizzle in egg and lemon juice mixture.
Pour soup over chicken and serve with fresh chopped parsley.