Chop the celery, including the leaves, quite small. Peel and chop the onions and chop the apple. Place them in a bowl, cover them with some water and the lemon juice until ready to use.
Heat the oil in a medium–large soup pot. Stir in the celery and onions, put the lid on and leave on a low heat for 5–10 minutes until the vegetables have started to soften. Add the apples, water and lemon juice. Stir in the salt and pepper and add the fresh thyme. Pour in the vegetable stock.
Bring to the boil then cover and simmer for 25 minutes. Add the chestnuts and simmer for a further 5 minutes. Allow to cool, add the milk and liquidise the soup. It can be coarse or smooth and creamy depending on your taste.