Whisk eggs and milk. Season with salt, pepper and nutmeg. Wash the herbs, dab dry, chop finely and stir into the egg mixture. Grease a flat ovenproof dish and pour in the egg mixture.
Cover the mould in a pot in a bain-marie and let it simmer at low heat for about 30 minutes. Clean and wash the mushrooms and cut them into fine slices. Bring beef stock to the boil. Turn out the eggs from the mould and cut into small cubes.
Add the mushrooms to the stock. Pour soup into soup bowls and serve garnished with chervil leaves.