Add chicken, carrots, potatoes, celery, mushroom, salt, pepper, thyme and stock to a large saucepan. Boil, then simmer for 15 minutes.
Drain and reserve the liquid stock.
Add butter and onions to a large saucepan. Cook onions for 5-6 mins on a medium heat, until softened. Add the flour and mix using a balloon whisk. Add in a ladle of the reserved stock. Stir with the whisk until smooth. Repeat until stock is used. Pour in the milk and heat whilst stirring with the whisk until the soup thickens.
Add the chicken and veggies back in. Add the kale. Heat through and add in half the lemon juice. Taste and add more lemon juice and seasoning if required. Sprinkle with parsley, then serve.