Chop onion and garlic finely, then roast with a little butter until golden brown. Add quartered mushrooms and fry gently. Deglaze with verjuice after a short time.
Braise on low heat with the lid closed until the mushrooms are almost done. Remove lid and allow sauce to boil down.
Remove from heat. Stir egg yolk and cream until smooth, then add to the sauce and bring to the boil again shortly. Season to taste with pepper and salt.
Sprinkle with chopped parsley.
Goes well with tagliatelle or rice.