Melt butter in a pan a fry the onions gently for 5 minutes until soft but not coloured.
Add mushrooms and cook gently, stirring for 5 minutes.
Stir in the flour and cook the mix for 1 minute, stirring occasionally.
Remove from heat and gradually stir in the chicken stock.
Bring to boil, stirring, until the soup is thick.
Stir in the lemon juice and milk.
Season well with salt and pepper.
Cover and simmer for 25 minutes but stir occasionally and keep a close eye on it that it doesn't boil over.
Garnish with parsley and serve.
To freeze, allow soup to cool and pour into lidded containers. Use within 6 months. To store in the fridge, cover and use within 2 days. This soup serves 4 people and takes about 15 to 30 minutes to prepare.