Heat the butter in a saucepan. When foaming subsides, but before it gets brown add the leeks and celery and 1/4 cup of the chicken broth. Cover and simmer over low heat for 5 minutes; do not let it get brown. Add the chicken, skin side up along with bouquet garni and enough of the broth to just cover the chicken. Cover and simmer, over very low heat for 30 minutes or until chicken is just cooked through. Remove chicken from the broth; remove and discard skin and bones and cut chicken into chunks. Remove and discard bouquet garni.
Puree the liquid through a food mill and return it to the saucepan. Add remaining broth and boil it down until reduced to 4 cups. Season to taste with salt and pepper. Add lemon juice to chicken and set in microwavable dish.
Blend egg yolks and cream together. Very slowly ladle 1 cup of hot broth into the mixture. Return this tempered mixture to the soup and bring to under a simmer to thicken. Reheat chicken for a minute in microwave oven and distribute it among 4 soup plates. Ladle thickened broth over chicken and garnish with parsley. Serve over boiled potatoes or with bread