Onions peel and quarter. Wash the turkey breast and pat dry. Cook the meat, onions and bay leaves in plenty of boiling salted water at low heat for about 1 hour.
Wash the asparagus and cut off the woody ends. Cut asparagus into bite-sized pieces. Drain the carrots. Clean, wash and slice the mushrooms and sprinkle with 1 teaspoon of lemon juice.
Take the meat out of the stock and dice it. Pour the stock through a sieve. Melt the fat and fry the asparagus and mushrooms briefly. Deglaze with 1/2 litre stock and simmer for about 10 minutes. Whisk egg yolk and 4 tablespoons of cream.
Add the meat, carrots and remaining cream to the vegetables and bring to the boil briefly. Season with salt, pepper, sugar and lemon juice. Stir in sauce thickener, bring to the boil again. Alloy with the whisked egg yolk.
Garnish as desired with steamed carrot cubes, chervil and lemon wedges.